ACCUMULO-1920 should not close shared zookeeper session; clean up unused watcher list

Conflicts:
	server/src/main/java/org/apache/accumulo/server/monitor/Monitor.java
Merge branch '1.5.1-SNAPSHOT' into 1.6.0-SNAPSHOT

Conflicts:
	server/monitor/src/main/java/org/apache/accumulo/monitor/Monitor.java