Fix #129: use transaction ID from connection's own transaction.
Don't pass transaction ID to connection.rollback_transaction().

Feedback from @dhermes.
Removing keyword "type" from storage.acl.
Using more descriptive entity_type instead of type_.
Merge pull request #180 from tseaver/129-rollback_using_connection_xact

Fix #129: use transaction ID from connection's own transaction.
Merge pull request #156 from dhermes/fix-overloaded-keyword

Removing overloaded global "type" from storage.acl.