fix thread breakage via ghost-on-removal

implement ghost-on-removal, the solution to T590-thread-breakage.sh
that just adds a ghost message after removing each message.

It leaks information about whether we've ever seen a given message id,
but it's a fairly simple implementation.

Note that _resolve_message_id_to_thread_id already introduces new
message_ids to the database, so i think just searching for a given
message ID may introduce the same metadata leakage.