ACCUMULO-3314 Add a fair volume chooser to stabilize RewriteTabletDirectoriesIT

The RandomVolumeChooser creates failure potential for the test
due to its random nature. A volume chooser that is stable should
result in a stable test.
ACCUMULO-3315 Wait for the pool to terminate before continuing
Merge branch '1.6'

Conflicts:
	server/base/src/main/java/org/apache/accumulo/server/util/RandomizeVolumes.java