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-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.