MB-10954 Use needs_rebalance when deciding on failover warnings.

So that we (almost) always show a warning when rebalance button is
enabled. The difference is in the set of nodes we pass to the
function. For the sake of failover safety warnings we pass only active
nodes. This helps us avoid a duplication of information that's shown to
the user. Specifically, otherwise we'd show a warning when the user just
adds a node to a cluster without hitting rebalance.

Change-Id: Ib5627b51919db2e7c1077f18d74e1c78e755f76b
Reviewed-on: http://review.couchbase.org/38280
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Allow to search for trivially matching past vbucket maps.

Where by "trivial" I mean:

  - nodes sets of past map and current map are the same
  - tags might be renamed but if two nodes were on the same tag before
  renaming they must reside on the same tag after the renaming.

Change-Id: If8178faa868397c3241092266ab78e0e3a5b460a
Reviewed-on: http://review.couchbase.org/38281
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>