OAK-593: Segment-based MK

Go back to caching accessed strings and templates per segment.
With the larger segment size achieved through the SegmentWriter changes
this again makes more sense than the centralized record cache in AbstractStore.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1550321 13f79535-47bb-0310-9956-ffa450edef68