Revert "Message queue: expose `finalize` fn for atomic-unlocking handlers, fix `error` fn"

This reverts commit 7bae2bad3c826fd00bc04848fdc68693c48cb3ce.
Message queue: add message id to handler args
~Message queue: add `enqueue` option for treating locked messages as non-dupes

This gives us another mechanism for eliminating race conditions: much simpler
for API consumers (though slightly less flexible) than using `finalize` with
an atomic handler.