- define checkstyle configs in as few places as possible
- use checkstyle:check since it has defaults that match what we want (fail after logging to console)
- modules that have current failures set property checkstyle.failOnViolation to false.

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

For reference, other bindings don't appear to specify a `groupId` at this level
in their `pom.xml` files.

@shivam-maharshi says in a [comment][1]:

> The groupId here is the duplicate of the parent groupId. This is creating a
> problem in the maven build.

  [1]: https://github.com/brianfrankcooper/YCSB/commit/d4b1d247a31cf9fbf42b2e324b5b1cc27fdd9565#commitcomment-15030853

The memcached support was extracted from PR #98 by @jbellis, with cleanups to
bring it in line with current APIs and style guide.

This PR also addresses issue #326.