Fix nesting tdb_traverse in a transaction

Fix nesting tdb_traverse in a transaction

Calling tdb_traverse inside a transaction led to the transaction lock being
held indefinitely. This was caused by the tdb_transaction_lock/unlock inside
tdb_traverse: The transaction code holds the global lock at offset
TRANSACTION_LOCK. The call to tdb_transaction_lock does nothing because the
transaction_lock is already being held. tdb_transaction_unlock inside tdb_wrap
resets tdb->have_transaction_lock but does not release the kernel-level fcntl
lock. transaction_commit later on does not release that fcntl lock either,
because tdb->have_transaction_lock was already reset by tdb_transaction().

This patch does fix that problem for me. An alternative would be to make
tdb->have_transaction_lock a counter that can cope with proper nesting, maybe
in other places as well.

Volker