ACCUMULO-2212 Add ZooReaderWriterFactory

The o.a.a.server.zookeeper.ZooReaderWriter class is mostly copied into a new
ZooReaderWriterFactory class. Users of the factory can work directly with FATE
ZooReaderWriter instances, instead of those of the server type, which is no longer
needed. The logic for building instances from site configuration data is in the
factory.

As part of this change, the invocation handler used to retry ZK calls on connection
loss is refactored into a new RetryingInvocationHandler.
Merge branch '1.6.1-SNAPSHOT'
ACCUMULO-2212 Fix UTF8 constant in ZooReaderWriterFactory

The merge of ACCUMULO-2212 work from the 1.6.1-SNAPSHOT branch introduced the use of
Constants.UTF8, which does not exist in the master branch. This commit replaces that
constant with a locally defined one.