ConnManager should trim using a background worker

[vyzo]

Currently we trigger a trim on every incoming connection.
This has two effects:

• contention to trim, which could lead to overtrimming if connections are slow to close and we have a fresh connection notification right after the silence period; this effect has been observed in a relay.
• failure to trim, if the connections that push us over the high water mark all happen within the grace periood or the silence period, whichever is greater, then they will not get trimmed.

I suggest we add a background worker that periodically trims instead of the current algorithm that relies on connection notifications.
The downside is that we will need a Close method to stop the background goroutine.