* xdcr: Add an internal api to read translog operations between a sequence number range.
This api will be used later by the persistent task for the following index to pull data from the leader index.
The persistent task can fetch the global checkpoint from the shard stats for each primary shard of the leader index.
Based on the global checkpoint of the primary shards of the following index, the persistent task can send several
calls to the internal api added in this commit to replicate changes from follow index to leader index in a batched manner.
Feature consists of a shell of a persistant task which will later be used to inspect the index settings and apply curator like changes to the index (move from hot to warm, rollover, shrink etc.)
This commit utilizes the pluggable engine factory feature in core to
introduce a pluggable engine factory for XDCR. For now this is only a
skeleton implementation to proof out the pluggable engine factory
concept. Future work will implement a genuine following engine for XDCR.
Relates #2655
This commit introduces the container class for CCR functionality. Future
work will expose more specific CCR functionality to the X-Pack plugin
through this class.
Relates #2704
Martijn van Groningen
Colin Goodheart-Smithe
Jason Tedor