ACCUMULO-2104 ACCUMULO-2106 Close the MTBW before deleting the tables in RW

If we don't close the MTBW, we have the potential to have failures be retried after we delete the tables (in the
teardown of the Fixture). This is, of course, besides the fact that a long-running RW client will just leak resources
like mad.
Merge branch '1.6.0-SNAPSHOT'