Add epoll based backend

Summary:
This is an epoll based EventBase backend (derived from EventBaseBackendBase).
It has the same functionality as the libevent base one with some implementation differences.
The timers are managed by a timer fd - similar to the high res one available in newer versions of libevent.
We expose the epollfd to allow for monitoring by other epoll instances.

Reviewed By: ot

Differential Revision: D42281517

fbshipit-source-id: 7b097aa8d06bffc8b3f622c77e74a814dfb17570

folly/experimental/io/Epoll.h

@@ -0,0 +1,23 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#if defined(__linux__) && __has_include(<sys/epoll.h>)
+#define FOLLY_HAS_EPOLL 1
+#else
+#define FOLLY_HAS_EPOLL 0
+#endif