[System Test]: Handlers hung in deploying state

Not seen on 7.0.0-4669

Build couchbase-server-7.0.0-4073 contains eventing commit b036e1f with commit message:
: Lock the bucketmap before connecting to the bucket

Function can be deployed in 2 ways. From SettingsChangeCallback and TopologyChangeCallback. During rebalance in of a eventing node there can be a race between these 2 callbacks and 2 different function can watch bucket at a same time. Eventing will update the bucketmap with the 2nd function and 1st one won't be included in the bucketMap due to race between concurrent access to map. So when 1st function tries GetBucket it will fail and exit. But it will remain in bootstrapping list. Users can run into this issue when they have multiple functions against the same source bucket and they try to rebalance-in an eventing node.

System recovered after killing all the producer on eventing nodes 1 by 1 and rebalance passed 

logs:  http://supportal.couchbase.com/snapshot/80d6eeb26726b35ea7abfb43a3070ecb::1

Subsequent rebalance and lifecycle operations passed too.