Recovery & view change implementation + bug fixes.

See merge request !7

Overview
========
This PR implements the view change and recovery portion of IR. It also
fixes a couple of important bugs and introduces a new mechanism with
with to unit test IR.

What's New?
===========
This PR includes the following, in roughly descending order of
importance:

- The main contribution of this PR is the implementation of view changes
  and recovery. The implementation required some of the following
  things:
    - I introduce DoViewChangeMessage and StartViewMessage protobufs as
      well as protobufs for serializing records.
    - IR replicas have logic to periodically initiate a view change or
      initiate a view change upon recovery. They also have logic to
      handle DoViewChangeMessage and StartViewMessage messages.
    - IR replicas persist their view information, which is needed for
      recovery, using PeristentRegisters which persist data on disk.
    - When a client issues a PropseInconsistentMessage or
      ProposeConsistentMessage, the reply indicating whether the request
      has already been finalized. If it has, the clients respond
      appropriately.
    - For conensus requests, clients make sure that the majority of
      replies and majority of confirms come from the same view.
- I fixed a slow path/fast path bug in IR clients. Previously, after a
  timeout, clients would transition from the fast path into the slow
  path. But, they would not wait for a majority of responses from
  replicas. Instead, they would call decide on whatever responses they
  had at the moment.
- Previously, decide functions took in a set of replies, but they needed
  to take in a multiset of replies so that the decide function could do
  things like count the frequency of each reply.
- I introduced a new simulated transport ReplTransport. It's pretty
  neat. It launches a command line REPL that you can use to manually
  control the execution of the system. You decide what timers to trigger
  and what messages to deliver. Then, you can save your execution as a
  unit test to run for later. I used the ReplTransport a lot to debug
  the view change and recovery algorithms since a lot of the corner
  cases are hard to trigger with the existing transport layers. I added
  some ReplTransport unit tests for the lock server.
- I implemented Sync and Merge for the lock server. The lock server is
  now fully functional.
- I generalized timeout callbacks to error callbacks which are invoked
  whenever an error (not just a timeout) is encountered. This was
  necessary to nicely handle some of the failure scenarios introduced by
  view changes (e.g. a client gets majority replies and majority
  confirms in different views for a consensus request).
- I performed some miscellaneous cleanup here and there, fixing
  whitespace, changing raw pointers to smart pointers, stuff like that.

What's Left?
============
There are still some things left that this PR doesn't implement:

- Currently during a view change, replicas send their entire records to
  one another. I'm guessing there are more efficient ways to transfer
  records between replicas, similar to how VR has some optimization
  tricks for log shipping.
- I have not implemented Sync or Merge for TAPIR, only for the lock
  server.
- Clients do not notify replicas of view changes when they receive
  replies from older views. Similarly, a replica never detects that its
  in a stale view or requests a master record from replica in a higher
  view. It just eventually does a view change to stay up-to-date.
- None of the code has been profiled or optimized or anything like that.
  This PR focuses only on correctness, not performance.
- There could be more unit tests.

Added a gitignore to the project.

See merge request !1

Added debugging rule to Makefile.

See merge request syslab/tapir!2

Removed `goto fail` code with `unique_ptr`.

See merge request syslab/tapir!3

Previously, `UDPTransport::SendMessageInternal` dynamically allocated a
`char[]` and used a `goto fail` to make sure that it was properly
deleted. Something like:

```c++
    char *buf = new char[100];
    if (...) {
        ...
        goto fail;
    } else if (...) {
        ...
        goto fail;
    } else {
        ...
    }

fail:
     delete [] buf;
     return false;
```

Now, the array is stored in a `unique_ptr` so that it's properly
deallocated when the function returns, without needing the goto fail.

Added a debugging Makefile rule stolen from [1]. With this rule, you can
run make print-<VAR> to print out the contents of <VAR>. For example,

        make print-OBJS
        make print-PROTOOBJS
        make print-BINS

[1]: https://blog.melski.net/2010/11/30/makefile-hacks-print-the-value-of-any-variable/

Previously, directories like `.obj/` and files like `libtapir.so`
weren't being ignored by git. Now they are! This .gitignore ignores vim,
c++, and project specific stuff.

A couple small lockserver client improvements.

1. Previously, typing ` `, `lock`, or `unlock` into a lockserver
   client's REPL would cause the client to crash because of some
   `strtok` calls that were returning `NULL`. Now, this bug is fixed;
   `strtok` returns are checked to be `NULL`, and no input should crash
   the client.
2. Previously, typing in an unrecognized command into the lockserver
   client's repl would print `Unknown command.. Try again!`. Now, it
   also prints out the set of legal commands `Usage: exit | q | lock
   <key> | unlock <key>`. This makes it a bit easier for someone
   tinkering around with TAPIR for the first time (like me) to know what
   to type.

fix building on Apple clang

This reverts commit 4791cfd3.