Fix processEventsWhileBlocked() to handle PENDING_WRITE clients.

After the introduction of the list with clients with pending writes, to
process clients incrementally outside of the event loop we also need to
process the pending writes list.

src/server.h

@@ -1110,7 +1110,7 @@ int listenToPort(int port, int *fds, int *count);
 void pauseClients(mstime_t duration);
 int clientsArePaused(void);
 int processEventsWhileBlocked(void);
-void handleClientsWithPendingWrites(void);
+int handleClientsWithPendingWrites(void);
 int clientHasPendingReplies(client *c);
 void unlinkClient(client *c);