=con #3882 Defer watch in ClusterSharding until after recovery

* To avoid unnecessary and costly watch/unwatch to non-existing systems.
* This avoids the problematic scario revealed in ticket 3879
Merge pull request #2020 from akka/wip-3882-sharding-watch-after-recovery-patriknw

=con #3882 Defer watch in ClusterSharding until after recovery