feat(transport): Worker eviction (#185)

Adds max worker capacity of 500 to the worker pool,
and an automatic shutdown of idle workers.
Messages that would cause the capacity to be exceeded,
are deferred.

Additionally, ensures that the same worker
is not spawned by two tasks at the same time.

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

filtermail/src/snapshots/filtermail__transport__tests__smtp_send_mail_defer.snap

@@ -20,7 +20,7 @@ TRANSACTION 1
 > message
 > .
 < 250 OK (SMTP)
-421 Worker for this destination is busy
+421 Transport busy
 
 [filtermail-transport -> destination A]
 < 220 filtermail SMTP

filtermail/src/snapshots/filtermail__transport__tests__smtp_send_mail_worker_cap_exceeded.snap

@@ -0,0 +1,51 @@
+---
+source: src/transport.rs
+expression: "format!(\"TRANSACTION 1\\r\\n\\\n            [postfix -> filtermail-transport]\\r\\n{record_postfix_1}\\r\\n\\\n            [filtermail-transport -> destination A]\\r\\n{record_filtermail_1}\\r\\n\\r\\n\\\n            TRANSACTION 2\\r\\n\\\n            [postfix -> filtermail-transport]\\r\\n{record_postfix_2}\")"
+---
+TRANSACTION 1
+[postfix -> filtermail-transport]
+< 220 filtermail SMTP
+> LHLO postfix
+< 250-filtermail
+250-8BITMIME
+250 OK
+> MAIL FROM:<sender@here>
+< 250 OK
+> RCPT TO:<a1@localhost>
+< 250 OK
+> DATA
+< 354 End data with <CR><LF>.<CR><LF>
+> message
+> .
+< 250 OK (SMTP)
+
+[filtermail-transport -> destination A]
+< 220 filtermail SMTP
+> EHLO example.org
+< 250-filtermail
+250-8BITMIME
+250 OK
+> MAIL FROM:<sender@here>
+< 250 OK
+> RCPT TO:<a1@localhost>
+< 250 OK
+> DATA
+< 354 End data with <CR><LF>.<CR><LF>
+> message
+.
+< 250 OK
+
+
+TRANSACTION 2
+[postfix -> filtermail-transport]
+< 220 filtermail SMTP
+> LHLO postfix
+< 250-filtermail
+250-8BITMIME
+250 OK
+> MAIL FROM:<sender@here>
+< 250 OK
+> RCPT TO:<b1@[127.0.0.1]>
+< 250 OK
+> DATA
+< 421 Transport busy