MB-100 fix for rebalance getting stuck

need to trap exits during spawn_and_wait call
and do the proper cleanup after the call

Change-Id: I073b5f9c1b208fbbc2a88f6ca020b31ef203b20d
Reviewed-on: http://review.couchbase.org/35056
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>