Previously, collate feature would be executed on all shards of an index using the client,
this leads to a deadlock when concurrent collate requests are run from the _search API,
due to the fact that both the external request and internal collate requests use the
same search threadpool.
As phrase suggestions are generated from the terms of the local shard, in most cases the
generated suggestion, which does not yield a hit for the collate query on the local shard
would not yield a hit for collate query on non-local shards.
Instead of using the client for collating suggestions, collate query is executed against
the ContextIndexSearcher. This PR removes the ability to specify a preference for a collate
query, as the collate query is only run on the local shard.
closes#9377
7efc43db25