MB-9980: implemented graceful failover

We implement graceful failover by vbucket moves that move all active
vbuckets of given node into replica positions and promote replica into
into active position instead. Which is followed by normal failover.

The idea is that graceful failover will move all vbuckets out of node
first and therefore without any data loss.

Change-Id: I26dec270242155698364778c38a4f5e886768cdc
Reviewed-on: http://review.couchbase.org/34294
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>