Optimize very common index state transitions (rebalance)

Optimize 2 types of state transitions:

1) If a state transition consists only of moving partitions
between the passive and active states (or vice-versa), that
is, no new partitions are added nor existing partitions are
marked for cleanup, don't restart the updater (at the expense
of more complex logic to correct snapshots received from a
currently running updater).

2) If a state transition only adds new partitions to the
passive state (as ns_server's rebalance frequently calls),
don't restart the updater, and send the updater the new
list of passive partitions, so that it has a chance to
iterate over those partitions' content and index them in
the same run.

As long as the rebalancer in ns_server doesn't keep changing
its logic regarding state transitions all the time, this
provides a significant reduction of the indexing time. However
for at least some scenarios, there's still significant portion
of rebalance time not spent on indexing nor index compaction.