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/ssl.hpp

@@ -66,6 +66,8 @@ struct ssl_shutdown_op
     {
         BOOST_ASIO_CORO_REENTER(*this)
         {
+            self.reset_cancellation_state(net::enable_total_cancellation());
+
             BOOST_ASIO_CORO_YIELD
                 s_.async_shutdown(std::move(self));
             ec_ = ec;