Limit rate of MDNs

New ratelimiter module counts number of sent messages and calculates
the time until more messages can be sent.

Rate limiter is currently applied only to MDNs. Other messages are
sent without rate limiting even if quota is exceeded, but MDNs are not
sent until ratelimiter allows sending again.

src/lib.rs

@@ -93,6 +93,7 @@ mod dehtml;
 mod color;
 pub mod html;
 pub mod plaintext;
+mod ratelimit;
 pub mod summary;
 
 pub mod dc_receive_imf;