Search on a shard group while relocation final flip happens might fail

At the final stage of a relocation, during the final flip of the states, a search request might hit a node that would then execute it on a shard that has already relocated.

For this, we need to execute broadcast and search operations against initializing shards as well, but only as a last resort. The operation will be rejected if not applicable (i.e. IndexShard#searcher() checked for read allowed).

Note, this requires careful though about which failures we send back. If we try and initializing shard and it fails, its failure should not override an actual failure of an active shard.

Also, removed an atomic integer used in broadcast request and use a similar shard index trick we now have in our search execution.

closes #3427