MB-16277: Set vbucket on future master to pending
MB-16277: Set vbucket on future master to pending

Set vbucket state to pending during rebalance, in order to decrease
risk of being evicted.

Change-Id: I335fc8ce2b98655e15affe0bbc2e0ff1667c3b95
Reviewed-on: http://review.couchbase.org/57640
Tested-by: Anders Nygren <4f8bff4ce020de40cfee5db6ca7fd51acc06e5c4@couchbase.com>
Reviewed-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>

membase/ns_server,membase/ns_server,membase/ns_server,ceejatec/ns_server,ceejatec/ns_server,membase/ns_server,ceejatec/ns_server,ceejatec/ns_server,membase/ns_server,membase/ns_server,ceejatec/ns_server,ceejatec/ns_server