[FEATURE] Split receive_200 into its own method

It was an alias of `receive_answer` but `receive_answer` also stores the record set and the response time duration, which you probably don't want to do on every 200 received. Also simplify `wait_for_answer` and fix the documentation, which stated that these expectations are optional by default when they are not.

A couple of other noteworthy points...

Should `send_bye` automatically call `receive_200`? Under normal circumstances, a 200 should always be expected but perhaps someone would want to use Sippy Cup to test abnormal circumstances. Or maybe such a user should be expected to drop back to raw SIPp. I did include `okay` in my new `receive_message` method and `recv response: 200` in my new `send_dtmf` INFO stuff, although the latter really is necessary because it has to be done after every single digit.

The rtd stuff in `receive_answer` doesn't really do anything at present because we're not setting `start_rtd` anywhere. These timers can also be used for more than just answering. Greater control over this would be nice.