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