ACCUMULO-2433 Can't reliably assume we won't get stale data with >1 tserver in test.

Add some retries to make sure we don't get stale data from ZooCache if we happen to talk
to the other MAC tserver.
ACCUMULO-2433 More test retries if at first we don't succeed.
Merge branch '1.5.2-SNAPSHOT' into 1.6.0-SNAPSHOT

Conflicts:
	test/src/test/java/org/apache/accumulo/test/ShellServerTest.java
ACCUMULO-2541 Set seekSleepTime and not (next)sleepTime in ScannerIT

Setting a batchSize of 1 will cause a new ScanSession for each k/v returned.
This means that next() will never be called on SlowIterator and thus the
intended effect of waiting server side doesn't actually happen.