Add Partial snapshot state

Currently even if some shards of the snapshot are not snapshotted successfully, the snapshot is still marked as "SUCCESS". Users may miss the fact the there are shard failures present in the snapshot and think that snapshot was completed. This change adds a new snapshot state "PARTIAL" that provides a quick indication that the snapshot was only partially successful.

Closes #5792
Switch to shared thread pool for all snapshot repositories

 Closes #6181
[TESTS] Ignore SnapshotMissingException in snapshotWithStuckNodeTest

The retry mechanism in the transport layer might cause the delete snapshot request to be executed twice if the cluster master is closed while the request is executed. First time delete snapshot request is getting successfully executed on the old master and then it is retried on the newly elected master. When the new master tries to delete the snapshot - the snapshot no longer exists (since it was successfully deleted by the old master) and SnapshotMissingException is returned.
More Like This Query: Added searching for multiple items.

The syntax to specify one or more items is the same as for the Multi GET API.
If only one document is specified, the results returned are the same as when
using the More Like This API.

Relates #4075 Closes #5857