MB-6368: Move EventuallyPersistentEngine refs out of kvstore

Change-Id: I8ad285d4f668ecbf85456d89f7028a15d97a928e
Reviewed-on: http://review.couchbase.org/25479
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>