CDRIVER-3947 Use "hello" command when an API version is declared (#784)

* Use hello command for handshake with versioned API

* Support handling hello command responses

* Deprecate legacy hello terminology

* Test handling hello command responses

* Refactor internal methods to use hello terminology

* Change mock server to also handle hello

* Replace deprecated terminology in test expectations

* Use hello in tests

* Fix speculative auth responder

* Fix legacy hello command usage

* Add test for behaviour when server does not support hello command

* Use legacy hello in tests to avoid failures on old server versions

* Update SDAM spec tests

* Fix memory leak

* Fix wrong command expectations for handshake

* Refactor handshake autoresponders

* Fix wrong comment style

* Rename mongoc_server_description_hello_response

* Use method to retrieve hello response

* Improve deprecation declaration

* Rename methods for hello callbacks

* Fix various test issues

* Rename hello_response method