OAK-788: File backend for the SegmentMK

In FileStore.close(), clear out references to the buffers and trigger garbage collection in an attempt to drop the mmap() references to the dataNNNNN.tar files

Improve the SegmentMK fixtures

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1478285 13f79535-47bb-0310-9956-ffa450edef68