tdb: use tdb_nest_lock() for transaction lock.

Rather than a boutique lock and a separate nest count, use our
newly-generic nested lock tracking for the transaction lock.

Note that the tdb_have_extra_locks() and tdb_release_extra_locks()
functions have to skip over this lock now it is tracked.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>