Replace the request queue in the connection with a channel

[mzimbres]

Asio channels are multiple-producers and multiple-consumer channels and can be used to simplify the connection internals by replacing the std::queue and per-request allocation of a timer. See https://github.com/chriskohlhoff/asio/blob/master/asio/src/examples/cpp20/channels/mutual_exclusion_1.cpp

[mzimbres]

After inspecting a bit the implementation, it turns out that a channel can't be used to replace the internal queue because we need the ability to partition the queue and set priorities on requests to support reconnection with proper resp3 handshake.