ACCUMULO-3231 Remove unused imports; fix javadocs
ACCUMULO-898 Suppress warning for serial version on old TInfo

  Thrift generated sources always suppressed these warnings. However, it
  should be noted that if anybody had actually serialized this class, its
  computed serialVersionUID will not match. This might limit the usefulness of
  including this for any desired backwards-compatibility reasons.
ACCUMULO-3318 Update thrift RPC secure socket creation to default to TLS
Merge branch '1.6'
ACCUMULO-3304 Track assignment execution duration and warn when they take longer than some duration.

Introduce a new configuration value, defaults to 10min, which controls
the duration an assignment must be running for a warning to be
printed. The period of checking threads is always half of the value.
ACCUMULO-3304 Add a basic test for assignment warnings.
Merge branch '1.6'

Conflicts:
	core/src/main/java/org/apache/accumulo/core/conf/Property.java
	server/tserver/pom.xml
	server/tserver/src/main/java/org/apache/accumulo/tserver/TabletServer.java
	server/tserver/src/main/java/org/apache/accumulo/tserver/TabletServerResourceManager.java
ACCUMULO-3318 Remove extraneous javadoc tag without description
Merge branch '1.6'
ACCUMULO-1177 use a thread pool for tablet assignment, but limit recovery to a single thread
ACCUMULO-3320 Update thrift idl
ACCUMULO-3320 Add a new method to the master to get the active tservers
ACCUMULO-3320 Fetch the necessary info from the master and tservers in the GC
ACCUMULO-3320 Use the active WALs from each tserver to preclude WAL deletion
ACCUMULO-3320 Normalize the WAL using Path.
ACCUMULO-3320 Integration test to ensure that WALs are not closed when a tserver may continue to use it.

Also test for the converse that after a WAL is unused by a tserver, it is still closed.
ACCUMULO-3320 Some cleanup and add IT suffix to test name.
ACCUMULO-3320 Stabilize the test after a noticed failure.
ACCUMULO-3320 At "IT" suffix to integration test.
ACCUMULO-3320 Fix up whitespace formatting.