Make ConnectionSet processing round-robin

[eile]

Reported by @delyas: The current ConnectionSet::select processing favors the first connections, which causes timeouts when a set of clients pushes a lot of requests fast (mapping) onto a single node, which will then first serve the 'first' clients.

Processing this in a round-robin fashion will ensure that all clients make progress and should speed up the rcv thread since commands are fully read already.

[eile]

Attention needs to be paid to the >64 connection threads on Windows.