ACCUMULO-2422 Refine renewal of master lock watcher

The first commit for ACCUMULO-2422 succeeds in renewing the watch on another master's lock
node when needed. This commit refines the solution:

- The renewal was happening even after the master is able to acquire the lock. This led to a
  spurious log error message. This commit skips renewing the watch in that case.
- If the renewal returns a null status, meaning the other master's lock node disappeared, the
  master now immediately tries again to acquire the lock. This matches watch establishment in
  other areas.

A lot of logging at the trace level was added to ZooLock to assist future troubleshooting.
Merge branch '1.5.1-SNAPSHOT' into 1.6.0-SNAPSHOT

Conflicts:
	server/src/main/java/org/apache/accumulo/server/master/Master.java