beast support per-op cancellation

- websocket supports cancellation.
 - Iterating test for ws cancellation.
 - Only terminal cancellation is forwarded by default.
 - basic_stream supports cancellation.
 - supported cancellation is documented.
 - http cancellation additions.
 - Added cancellation_slot tests to http, utils and saved_handler.
 - Added post to write.cpp, to avoid SIGSEV in test.
 - Refresher describes cancellation in more detail.

include/boost/beast/websocket/impl/accept.hpp

@@ -266,6 +266,9 @@ public:
 
 // read and respond to an upgrade request
 //
+// Cancellation: the async_accept cancellation can be terminal
+// because it will just interrupt the reading of the header.
+//
 template<class NextLayer, bool deflateSupported>
 template<class Handler, class Decorator>
 class stream<NextLayer, deflateSupported>::accept_op