beast::asio improvements
* New enable_wait_for_async for safe cleanup
* New memory_buffer, a managed asio compatible buffer
* shared_handler improvements:
  - Can be 'empty' (no stored handler).
  - Default constructible as 'empty'.
  - Safe evaluation in bool contexts, false==empty
Remove unused BufferType
[WIP] Make IPAddress header-only
Remove obsolete classes:
* NamedPipe
* ReadWriteLock
* ScopedReadLock
* ScopedWriteLock
* LockGuard