memcached/ep-engine crashes in flusher or other paths when it receives a shutdown message from ns-server

this case could occur in many places

1- when node is waming up and ns-server sneds a shutdown command to delete the bucket during warmup
2- when node is warming up ( a failed over node ) ns-server sends a shutdown command to delete the bucket
3- when a node which was rebalanced out but for some reason memcached is still doing sth , ns-server sends a shut down command

scenario #2 is very very common and in large environments where warm up takes 8 hours or so user will keep retrying the rebalance button and it wont succeed unless user manually kills the memcached process manually by running kill command.

in general ep-engine needs to abort instead of crashing
on the other hand during normal shutdown , when ns-server sends a command to ep-engine to shut down . ep-engine should wait until all items are flushed and then shutdown.

seems like we need to differentiate a command that says shut down gracefully or shut down with force.

some of the bugs :

http://www.couchbase.com/issues/browse/MB-7110
http://www.couchbase.com/issues/browse/MB-7263