Make send thread-safe with asio strand

[alesapin]

I'm not sure that this one is the best possible solution. This check was introduced in #106, but the peer's busy flag doesn't cover all possible ways to call send in asio_service. Fixes #169.

[alesapin]

Still have a race condition in asio https://gist.github.com/alesapin/3f09d3fad35bb95f2f2753be7b859022 :(

[greensky00]

Please refer to my comment
#169 (comment)

Multiple messages can't be sent to the same peer at the same time by design.

[alesapin]

As discussed in #169 these changes don't help and only hide the error.

[nicelulu]

Multiple messages can't be sent to the same peer at the same time by design.

@greensky00 Perhaps this restriction can be relaxed for the following request? auto_forwarding_= true and follower's request to the leader and the requested msg_type is msg_type::client_request.