Refactoring libboostasio (part 2)

[paolopas]

This PR continues the factorization of the #550.

Note

This is the second child, the code is branched from #553 (first child), so this one accumulate the changes.

Each commit has been checked for regressions.

• 37e31ba clean up
• 40ed673 get_dispatch_wrapper finally replaced with the new make_handler,
  I tried to keep the changes to the bare minimum.
  • The new handler is faster; previously, a callback required two boost::bind (one of which was copied when passing parameters) plus one lambda. Now, all you need is a wrapper around a member pointer (std::mem_fn, which is then forwarded and not copied), a std::bind, and a lambda.
  • Smart pointers guarding against any potential dangling pointers are managed more simply and consistently, freeing target callback methods from the responsibility of dealing with them.
  • All callbacks now have the same signature, and the methods that generate them could even be replaced by a macro.
  • The last but not the least everything is now easier to understand (that is, develop and maintain); readability counts
  • std::function is derived from the analogous boost (same for std::bind), to avoid mixing std and boost I have replaced the boost::bind and boost::function with the equivalents from the standard library. This also helps with Support of ASIO standalone edition #381
• 5090d0f Added a bottom note about a race condition involving _read_pendingand _write_pending flags. This issue will be automatically fixed along with Boost handler doesn't work with SSL #424 (already tested), the note serves as a warning.

At this point the handler is just more robust and cleaner than before, from the next PRs I will start adding features and bug fixes.