ACCUMULO-2635 Add ZooCacheFactory and integrate across code

This commit introduces a ZooCacheFactory class to centralize and maximize reuse of
ZooCache instances and to facilitate testing. The caching mechanism formerly in ZooCache
was moved to the factory; also, the separate caching mechanism in ServerClient was removed
in favor of the factory.
Merge branch '1.6.1-SNAPSHOT'

Conflicts:
	core/src/main/java/org/apache/accumulo/core/client/ZooKeeperInstance.java
ACCUMULO-2635 Replace Constants.UTF8 in merged ZooKeeperInstanceTest