feat: read SMTP recipient limit from relay IMAP metadata

Remove is_chatmail flag, and rely on IMAP metadata
advertising the recipients limit (usually 1000),
falling back to 50 (or fewer in some exceptional cases)
just as before when is_chatmail was false.

Also, server metadata is now keyed per transport,
which in the future eases collecting ICE servers
from all relays (instead of just any first relay).

src/transport/transport_tests.rs

@@ -253,7 +253,6 @@ async fn test_empty_server_list() -> Result<()> {
     assert_eq!(loaded.provider, Some(*provider));
     assert_eq!(loaded.imap.is_empty(), false);
     assert_eq!(loaded.smtp.is_empty(), false);
-    assert_eq!(t.get_configured_provider().await?, Some(*provider));
 
     Ok(())
 }