Buffer IMAP client writes

async-imap does not do its own buffering, but calls flush() after
sending each command. Using BufWriter reduces the number of write()
system calls used to send a single command.

Note that BufWriter is set up on top of TLS streams, because
we can't guarantee that TLS libraries flush the stream before
waiting for response.

src/imap/session.rs

@@ -6,6 +6,7 @@ use async_imap::types::Mailbox;
 use async_imap::Session as ImapSession;
 use async_native_tls::TlsStream;
 use fast_socks5::client::Socks5Stream;
+use tokio::io::BufWriter;
 use tokio::net::TcpStream;
 use tokio_io_timeout::TimeoutStream;
 
@@ -33,12 +34,17 @@ pub(crate) trait SessionStream:
     fn set_read_timeout(&mut self, timeout: Option<Duration>);
 }
 
-impl SessionStream for TlsStream<Box<dyn SessionStream>> {
+impl SessionStream for Box<dyn SessionStream> {
+    fn set_read_timeout(&mut self, timeout: Option<Duration>) {
+        self.as_mut().set_read_timeout(timeout);
+    }
+}
+impl<T: SessionStream> SessionStream for TlsStream<T> {
     fn set_read_timeout(&mut self, timeout: Option<Duration>) {
         self.get_mut().set_read_timeout(timeout);
     }
 }
-impl SessionStream for TlsStream<Pin<Box<TimeoutStream<TcpStream>>>> {
+impl<T: SessionStream> SessionStream for BufWriter<T> {
     fn set_read_timeout(&mut self, timeout: Option<Duration>) {
         self.get_mut().set_read_timeout(timeout);
     }
@@ -48,7 +54,7 @@ impl SessionStream for Pin<Box<TimeoutStream<TcpStream>>> {
         self.as_mut().set_read_timeout_pinned(timeout);
     }
 }
-impl SessionStream for Socks5Stream<Pin<Box<TimeoutStream<TcpStream>>>> {
+impl<T: SessionStream> SessionStream for Socks5Stream<T> {
     fn set_read_timeout(&mut self, timeout: Option<Duration>) {
         self.get_socket_mut().set_read_timeout(timeout)
     }