mailbox tubes

Most tubes exist as long as jobs exist in them or a client is watching or using them. Mailbox tubes exist only while a client is watching. If no clients are watching, the tube and all jobs in it vanish.

A common use for this is to indicate completion of a job by a "reply job" sent to a mailbox tube.

Such tubes can be marked as special by prefixing the name with a special char. I think "~" would work fine as that char.

No source files, tests, or entry points are named. Begin by locating the code that creates, watches, and removes tubes, then trace how jobs are retained; the proposal is done when a prefixed mailbox tube exists only while a client watches and its queued jobs disappear when the last watcher leaves.

Written by the indexing model from the issue text.