Created asio::experimental::awaitable_specific_ptr

[corporategoth]

@chriskohlhoff I am trying to bring coroutines in parity with threads somewhat, specifically, trying to replicate boost::thread_specific_ptr, but for coroutines (awaitables).

This can be hacked externally with a lot of effort, including having to create your own executor, and make sure you use it everywhere, and then use a different awaitable class and use_awaitable. But it's hacky and relies on the user to do a lot of things, and remember to do them always. It would be like if boost::thread_specific_ptr only worked if they use my_thread instead of std::thread to launch their thread.

This creates an implementation of awaitable_specific_ptr that works with coroutines, and is automatic for any awaitable, so the user doesn't have to do anything except USE awaitable_specific_ptr - it's automatic.

Given there is no real "awaitable ID" that is externally accessible to help implement this externally, this would be a large boon for being able have an equivalent of thread-specific storage (for example for use with logging contexts, etc).