Refactoring libboostasio (part 1)

[paolopas]

This PR is the result of the initial factorization of #550 (hence it's first child).

After each commit included in the PR I checked that there were no regressions, having more revisions will make the review easier. If @zerodefect thinks it's useful I can merge them into a single revision for the merge.

• 062d92f Update and clean up documentation (README and code)
• 6be5e1b Better example test, fixed io_contex::strand header, comments cleanup
• 0291bd2 End of virtual inheritance abuse (why this?)
• 5b32219 Abandon the deprecated deadline_timer
• 3c0c4d6 Abandonment of deprecated null_buffers, this made it necessary to convert the async_read_some/async_write_some to async_wait and consequently also the callback prototype (fortunately); Furthermore, these methods never return a boost::asio::error::would_block error if the socket is non-blocking.
• f7c4726 At this point the handlers are starting to become a little more homogeneous, but the real change is yet to come.