tdb: don't suppress the transaction lock because of the allrecord lock.
tdb_transaction_lock() and tdb_transaction_unlock() do nothing if we
hold the allrecord lock. However, the two locks don't overlap, so
this is wrong.
This simplification makes the transaction lock a straight-forward nested
lock.
There are two callers for these functions:
1) The transaction code, which already makes sure the allrecord_lock
isn't held.
2) The traverse code, which wants to stop transactions whether it has the
allrecord lock or not. There have been deadlocks here before, however
this should not bring them back (I hope!)
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>