tdb2: allow readonly changes even while holding locks.

tdb2: allow readonly changes even while holding locks.

This happens in SAMBA with the TDB_VERSION1, presumably due to a
read-only traverse nested inside a normal traverse (since it doesn't
occur without TDB_VERSION1).

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
(Imported from CCAN commit 24e5ddb143fb5e79112649472258f5da67cc7362)

Autobuild-User: Rusty Russell <rusty@rustcorp.com.au>
Autobuild-Date: Tue Sep 20 09:35:10 CEST 2011 on sn-devel-104