closes #340

[core accumulo couchbase memcached] Fixing squid:S1155 - Collection.isEmpty() should be used to test for emptiness

Replaces numeric status codes with a canonical set of Status objects,
each with a short name and description.
Bindings with more specific errors (e.g., timeouts) return additional
statuses.

This changes the default output from messages like:

   [UPDATE], Return=0, 511

To:

   [UPDATE], Return=OK, 511

Again, a lot of whitespace changes and the normal checkstyle type errors.

2 items of note:

1. Deprecated QueueElement since it is not used.
2. Moved some of the exception handling to common locations to remove
   empty catch blocks.

* Increment Accumulo Version to 1.6.0
* Fix accumulo dependency versions
  - Harmonize guava version for Accumulo.
  - Automatically manage slf4j version.
* Clean up depr. code
* Clean up return codes and error logging

Make the batchwriter's number of threads configurable.

clean up dead code in client.

Based on YCSB++

http://www.pdl.cmu.edu/ycsb++/