Fix dangling goroutine on session expiration

The timer channel may already have been drained by `case <-timer.C`. In
this case, we need not to drain it again or we'll block forever.

To fix this, stop draining the timer channel. Since we're not going to
use the timer again anyway, it should be fine.

session.go

@@ -237,9 +237,7 @@ func (sm *SessionManager) Put(username, password string) (*Session, error) {
 			}
 		}
 
-		if !timer.Stop() {
-			<-timer.C
-		}
+		timer.Stop()
 
 		s.locker.Lock()
 		if s.imapConn != nil {