tdb: add tdb_freelist_merge_adjacent()

tdb: add tdb_freelist_merge_adjacent()

This is intended to be called to reduce the fragmentation in the
freelist. This is to make up the deficiency of the freelist
to be not doubly linked. If the freelist were doubly linked,
we could easily avoid the creation of adjacent freelist entries.
But with the current singly linked list, it is only possible
to cheaply merge a new free record into a freelist entry on the left,
not on the right...

This can be called periodically, e.g. in the vacuuming process
of a ctdb cluster.

Signed-off-by: Michael Adam <obnox@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>