[Dovecot] lmtp proxy logging

The logging on lmtp and lmtp proxy is pretty limited from what I can see. It seems to handle errors, Connect, Disconnect, and in the case of lmtp delivery, it logs where an email is saved to. The lmtp may be enough, "connect, saved user, saved user..., disconnect", but I was curious if it is worth while to add more info logging for the proxy, primarily which recipients are sent to which proxy. I was thinking of local patching it, but I'll generate up something more inline with official code if it is desired.

My thought is to show 1 entry for each recipient, and the destination server chosen. If I recall correctly, the proxy code doesn't actually listen in on the conversation, so logging results would probably complicate the code.

Jack