MIDI-CI: request/retry timeout manager

[atsushieno]

(taken from #56)

These items are subject to timeout, and we do not have any timeout manager so far:

• requestID: recycled within 0-127, need to keep which is active
• subscriptions: they could stay forever
• discovery (after 3 seconds we could drop any DiscoveryReply. We do not have to though.)

We would also need request pending queue, or immediately send retry. There should be some policy on how to deal with them. Depending on it, we will have to send ACK (which is currently not in use).