feat(transport): Destination worker pool

Implements a per-destination worker pool,
so that connections to the same destination
are not parallelized, but instead queued.
If a queue is full, new messages are immediately
deferred, before mail data is sent from postfix.

Closes: #141

Signed-off-by: Jagoda Ślązak <jslazak@jslazak.com>

filtermail/src/main.rs

@@ -33,6 +33,7 @@ pub(crate) mod message;
 pub(crate) mod openpgp;
 pub(crate) mod outbound;
 pub(crate) mod smtp_client;
+mod smtp_responses;
 pub(crate) mod smtp_server;
 mod tls;
 mod transport;
@@ -50,8 +51,6 @@ use std::process;
 use std::str::FromStr;
 use std::sync::Arc;
 
-const ENCRYPTION_NEEDED_523: &str = "523 Encryption Needed: Invalid Unencrypted Mail";
-
 #[derive(Debug, Copy, Clone, PartialEq, Eq)]
 enum Mode {
     Outgoing,