This commit enables completion suggester to return documents
associated with suggestions. Now the document source is returned
with every suggestion, which respects source filtering options.
In case of suggest queries spanning more than one shard, the
suggest is executed in two phases, where the last phase fetches
the relevant documents from shards, implying executing suggest
requests against a single shard is more performant due to the
document fetch overhead when the suggest spans multiple shards.
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