OAK-1634: After crash, segment persistence is broken with failures in java.nio classes (with v0.19)

Add more sanity checks and a mechanism for automatically rewinding to a
previously committed revision if the latest one can't be accessed.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1583352 13f79535-47bb-0310-9956-ffa450edef68