ACCUMULO-2489 Fixes race condition in TableConfiguration where NPE may occur.

Make invalidateCache much more efficient by calling clear on ZooCache
instead of creating a new one.
Merge branch '1.5.2-SNAPSHOT' into 1.6.0-SNAPSHOT

Conflicts:
	server/base/src/main/java/org/apache/accumulo/server/conf/TableConfiguration.java
Merge branch '1.6.0-SNAPSHOT'