dbwrap: add dbwrap_tdb2 backend

dbwrap: add dbwrap_tdb2 backend

This backend can be used untill ctdb knows about real
transactions.

It stores a master tdb in a shared location and a readonly copy
on the local harddisk. Reads are always on the local tdb
and writes always on both. Change notify messages are send
to all message context, which ask for them. With the notifies
it's possible to just update the changed records, instead of
copying all records (which is the fallback).

You need to configure:

dbwrap:use_tdb2=yes
dbwrap_tdb2:master directory=/some/shared/path
dbwrap_tdb2:local directory=/var/lib/samba

metze
(cherry picked from commit aa6230de0d5f1875aa8c12c4fc017d3a40f90890)