ACCUMULO-1376 Shell verifies before dropping user

Signed-off-by: Josh Elser <elserj@apache.org>
ACCUMULO-1376 Add a unit test that ensure that the user is prompted w/o the force option when dropping a user
ACCUMULO-2899 Correctly handle older internals.

Make sure that we behave correctly when our cluster has been upgraded 1.4 -> 1.5 -> 1.6

* dfsloggers need to track the cq used to create them so they can be used to issue putDeletes
* rewrite 1.4 WAL paths to the top level wal directory
* adjust recovery path to compensate for 1.4 recovered wals
* fix GC to properly handle upgraded WALs

Author: Mike Drob <mdrob@cloudera.com>
Author: Sean Busbey <busbey@cloudera.com>
Merge branch '1.6.1-SNAPSHOT'

Conflicts:
	server/tserver/src/main/java/org/apache/accumulo/tserver/tablet/Tablet.java