Create a sendmail command or smtp listener to capture generated email

[brong]

We need this for various tests:

• sieve redirects
• JMAP EmailSubmission
• Calendar alarms (currently done by watching notifyd and using the notifyd method, but in theory we should be testing the full stack including email sending)

[rsto]

I recently added a simple SMTPServer to Cassandane, based on RobMs' XMTPServer (which is vender for Cassandane here). It works out of the box with the smtpclient API in Cyrus.

The begin_data and end_data routines should be a start. We fork the SMTP server once, so there's the question how to map the data to the respective originating test: For JMAP, we could get away with a custom envelope. But we might also want to add a more generic solution to either Cassandane or even the new smtpclient layer in Cyrus.