Simplify ChannelTcp

[emgre]

There are multiple issues to me in this class:

• It does too much things. Currently, it does the following:
  • Act as a funnel, taking multiple requests and sending them one by one (or a few at a time, see #3)
  • Builds the MBAP message (includes buffer creation and writing the message using MbapMessage::build_message).
  • Handle sending the received data to the parser and reseting it when something goes wrong.
  • Handle timer management for the request that was sent.
  • Handle transaction id increment (although this is a small responsibility).
• The copying of IRequest in ChannelTcp::send_request is horrible. The temporary shared_ptr to pass it to the lambda feels wrong.
• The behaviour of the object after it was shutdown is undefined. I think it can revive the connection even if the asio threads are joined. I also think this issue affects opendnp3 btw.

A first avenue would be to separate the funneling from the message build/parser submission. It would clarify the code (I hope...)

[jadamcrain]

Ok. I trust your judgement on how best to do this. I assume the notes are mostly for your own reference.

I can have a look at the implementation sometime this week and see if there's any suggestions I can make.