Bug 2187: Don't close over internal state in ShardManager

Bug 2187: Don't close over internal state in ShardManager

For AddShardReplica, we use the ask pattern for the FindPrimary and
AddServer messages. However in the OnComplete callbacks we're closing
over internal state which isn't safe since the callback will be notified
outside of the actor's execution context which may result in concurrent
mutation of internal state. Therefore I added internal messages that are
sent to self in the callbacks.

Change-Id: I1f6662a4e473749925046f127cad868e54b761a2
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>