ACCUMULO-1935 Sufficiently increase timeouts in attempt to avoid missed notifications.
ACCUMULO-1908 Lots of small fixes that make ShellServerTest a little more stable.

Try to isolate each test using new tables each test. Increase timings, account
for caching, better use of JUnit asserts for meaningful failures, and better
helper methods (countFiles).
ACCUMULO-2331 Actually invalidate the cache in TableConfiguration when requested.

TableConfiguration previously didn't override invalidateCache, which means that
the implementations that used it and the invalidateCache method, were not providing
the semantics that were intended. Fix the case in SimpleTest that outlined this issue,
removing the additional poll/check loop as it was both broken and unnecessary with
proper API implementation.
Merge branch '1.5.1-SNAPSHOT' into 1.6.0-SNAPSHOT

Conflicts:
	server/base/src/main/java/org/apache/accumulo/server/conf/TableConfiguration.java
	test/src/test/java/org/apache/accumulo/test/ShellServerIT.java
Merge branch '1.6.0-SNAPSHOT'
ACCUMULO-2021 Replace ArgumentChecker with Guava Preconditions

The ArgumentChecker offers some of the same argument checks as the Preconditions class
in Guava. This commit eliminates ArgumentChecker and replaces its uses with calls to
Preconditions, particularly Preconditions.checkArgument(). The Validator class inside
ArgumentChecker was also promoted to a regular class with its own unit test.