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