OAK-1159: Backup and restore

Add {{SegmentStore.isInstance()}} for checking whether a record is of
a given type and from a given store. This way we can replace the
instanceof checks that so far assumed that there was just a single
SegmentStore in the system. That assumption is not true when we're doing
backups.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1546123 13f79535-47bb-0310-9956-ffa450edef68