Handle get_sqe failures, don't die with "will fall out of sync" in epoll_manager

Problem is that in recent kernels io_uring may return completions BEFORE
clearing the submission queue. I.e. for example its capacity is 512, there
were 512 requests, one of them completed, so when the request completion is
processed the queue "should have" 1 free slot. But sometimes it doesn't because
io_uring doesn't always clear the submission queue before sending CQE :-/

src/epoll_manager.h

@@ -11,6 +11,8 @@
 class epoll_manager_t
 {
     int epoll_fd;
+    bool pending;
+    ring_consumer_t consumer;
     ring_loop_t *ringloop;
     std::map<int, std::function<void(int, int)>> epoll_handlers;
 public: