Added Cluster Stats API

Closes #4460
Improve allocation of unassigned shards with early termination

When we allocate unassigned shards we can terminate early for some
shards like if we already tried to allocate a replica we don't need
to try the same replica if the first one got rejected. We also
can check if certain nodes can't allocate any primaries or shrads
at all and take those nodes out of the picture for the current round
since it will not change in the current round.
Randomize AllocationDecider order in tests