bpo-33654: Support BufferedProtocol in set_protocol() and start_tls() (GH-7130)

In this commit:

* Support BufferedProtocol in set_protocol() and start_tls()
* Fix proactor to cancel readers reliably
* Update tests to be compatible with OpenSSL 1.1.1
* Clarify BufferedProtocol docs
* Bump TLS tests timeouts to 60 seconds; eliminate possible race from start_serving
* Rewrite test_start_tls_server_1