Updates `add_timer` to return a new opaque TimerID object that can later
be passed to `cancel_timer` to cancel an existing timer.
Also adds timer tests, which was omitted (except for one in the tagged
threads section), along with a new test for timer deletion.
Storage server, in particular, needs to disable pubkey-based routing on
its connection to oxend (because it is sharing oxend's own keys), but
wants it by default for SS-to-SS connections. This allows the oxend
connection to turn it off so that we don't have oxend omq connections
replacing each other.
