store: reset backoff on success

Fixes: zulip#554

Author: chrisirhc

lib/model/store.dart

@@ -694,7 +694,6 @@ class UpdateMachine {
             assert(debugLog('Transient error polling event queue for $store: $e\n'
                 'Backing off, then will retry…'));
             // TODO tell user if transient polling errors persist
-            // TODO reset to short backoff eventually
             await backoffMachine.wait();
             assert(debugLog('… Backoff wait complete, retrying poll.'));
             continue;
@@ -716,6 +715,9 @@ class UpdateMachine {
       if (events.isNotEmpty) {
         lastEventId = events.last.id;
       }
+
+      // On success, reset the backoff.
+      backoffMachine.reset();
     }
   }