Bug 8385: Fix testMultipleRegistrationsAtOnePrefix failures
The test quickly creates/removes the prefix shard in iterations which
can result in an InvalidActorNameException if the shard actor from the prior
iteration hadn't been destroyed yet. To alleviate this I modified the
removal in the ShardManager to utilize Patterns.gracefulStop to store the
Future and block a subsequent create until the Future completes.
Change-Id: Ica98de3cc17c2d87195840bdf052d81ed3b9dd10
Signed-off-by: Tom Pantelis <tompantelis@gmail.com>