Fix sporadic ShardManagerTest failures

Fix sporadic ShardManagerTest failures

Some of the tests fail sporadically. Most were alleviated by:

  - using tell on an actor rather than calling receiveCommand directly
  - using the normal fork/join dispatcher for creating TestActors instead
    of the default CallingThread dispatcher.

After the changes the tests ran over 200 times successfully.

Change-Id: Ib2c7c3b6dace9e89dff54eccc58a2b8aabad75de
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>
(cherry picked from commit 2f6d96da89035d5aec78c90bce5065d2f202a515)