Replaces asio::post by async_immediate for immediate completions

[anarthal]

Adds a missing include to except.hpp

[anarthal]

async_immediate is the current recommended way of handling async immediate impletions AFAIK. It's equivalent to asio::post, but will handle the immediate_executor mechanism (i.e. the "remap post to dispatch") transparently.

[cppalliance-bot]

An automated preview of the documentation is available at https://37.http-io.prtest2.cppalliance.org/index.html