Add public API to SMTPClient to control state transitions

[itamar]

SMTPClient's state machine cannot be controlled by subclasses without setting private attributes; the attached patch switch to a public API for setting this.

Alternatively, maybe it's easier to just throw SMTPClient out and start from scratch.

[itamar]

Patch to twisted.mail.smtp

[itamar]

I am uncertain whether these are minimal enough chances that the current unit tests cover it, and don't have time to check right now.

[exarkun]

Alternatively, maybe it's easier to just throw SMTPClient out and start from scratch.

This is attractive. (Of course, the order we go in is "start from scratch" and then "throw SMTPClient out".) The overall structure of SMTPClient is extremely cumbersome and the interface is obtuse.