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/context.rs

@@ -3,7 +3,7 @@
 use std::collections::{BTreeMap, HashMap};
 use std::ffi::OsString;
 use std::ops::Deref;
-use std::time::{Instant, SystemTime};
+use std::time::{Duration, Instant, SystemTime};
 
 use anyhow::{ensure, Result};
 use async_std::{
@@ -22,6 +22,7 @@ use crate::key::{DcKey, SignedPublicKey};
 use crate::login_param::LoginParam;
 use crate::message::{self, MessageState, MsgId};
 use crate::quota::QuotaInfo;
+use crate::ratelimit::Ratelimit;
 use crate::scheduler::Scheduler;
 use crate::sql::Sql;
 
@@ -55,6 +56,7 @@ pub struct InnerContext {
     pub(crate) events: Events,
 
     pub(crate) scheduler: RwLock<Option<Scheduler>>,
+    pub(crate) ratelimit: RwLock<Ratelimit>,
 
     /// Recently loaded quota information, if any.
     /// Set to `None` if quota was never tried to load.
@@ -186,6 +188,7 @@ impl Context {
             translated_stockstrings: RwLock::new(HashMap::new()),
             events: Events::default(),
             scheduler: RwLock::new(None),
+            ratelimit: RwLock::new(Ratelimit::new(Duration::new(60, 0), 3.0)), // Allow to send 3 messages immediately, no more than once every 20 seconds.
             quota: RwLock::new(None),
             creation_time: std::time::SystemTime::now(),
             last_full_folder_scan: Mutex::new(None),