Fix for connection cleanup

[eddyashton]

Combination of recent connection cleanup changes were dangerous, as noticed by the ASAN build. We were modifying a collection while iterating over it:

for (auto& [id, idle_time]: idle_times)
{
  ...
  stop(id);
  ...
}

bool stop(ConnectionID id)
{
  ...
  idle_times.erase(id);
  ...
}

We could potentially drop this erasure entirely, I think it's a safe but unnecessary step. But to be a little safer, less likely to spam logs in an edge case, I've retained it - callers of stop(id) now call idle_times.erase(id) themselves.

[maxtropets]

We were modifying a collection while iterating over it:

Is it possible we still modify it simultaneously from both on_timer() and message dispatch?

[eddyashton]

We were modifying a collection while iterating over it:

Is it possible we still modify it simultaneously from both on_timer() and message dispatch?

No - these both happen on the same thread, as part of libuv's dispatch loop.