[ENGINE] Move engine lifecycle store reference to EngineHolder

This commit moves the engines reference to the store out of the actual
implementation into the hodler since the holder manages the actual lifcycle.
Engine internal references like per searcher or per recovery are kept inside
the actual implemenation since the have a different lifecycle.

this caused failures like this: 

http://build-us-00.elasticsearch.org/job/es_core_master_metal/5917/
can we add a comment to the PR description indicating this issue as well?