Expire idle matrix-side ghosts out of memory

[leonerd]

Currently, every matrix-side ghost user maintains presence state for all of time. If you don't restart the bridge for a while, this constant accumulation of users starts creating even more presence traffic in the HS.

It would help to reduce the amount of meaningless presence spam, if we start expiring these ghosts if they haven't been active for a while; maybe a day or so. The users already keep last-active timestamps, for exporting the active-users metrics, so it should be simple enough to expire based on these.

[leonerd]

/cc @erikjohnston This should help reduce gitter-user presence spam