ACCUMULO-2342 Sleep after we modify a constraint.

We have to wait for the watcher on zk to trigger and propagate down to
the constraint observer so that the mutations won't be incorrectly
rejected and fail these unit tests.
Merge branch '1.5.1-SNAPSHOT' into 1.6.0-SNAPSHOT

Conflicts:
	proxy/src/test/java/org/apache/accumulo/proxy/SimpleProxyIT.java