tdb: use tdb_nest_lock() for active lock.

Use our newly-generic nested lock tracking for the active 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>
(Imported from commit 4738d474c412cc59d26fcea64007e99094e8b675)