Turn smtp_peer_revdns into a repeated field.

A host may have multiple PTR records, so we should capture them all.
Change the API a bit (pass a Reader into Data, add ETRN).
Support returning multiple lines of things using Respond and friends.
Add some code to handle connections being set up.

Store all available data (so far) about the peer in the protobuf which
is passed along as userdata.