Implemented a TimerQueue class and a Timer class

The Windows implementation of the Timer class (in timer.h) used Timer
Queues. This brings in implementation for a mostly Windows complient
TimerQueue class to work with the Timer class.

internal/platform/implementation/linux/timer.h

@@ -0,0 +1,56 @@
+// Copyright 2021 Google LLC
+//
+// 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
+//
+//     https://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.
+
+#ifndef PLATFORM_IMPL_LINUX_TIMER_H_
+#define PLATFORM_IMPL_LINUX_TIMER_H_
+
+#include <memory>
+
+#include "absl/base/thread_annotations.h"
+#include "absl/synchronization/mutex.h"
+#include "internal/platform/implementation/timer.h"
+#include "internal/platform/implementation/linux/submittable_executor.h"
+#include "internal/platform/implementation/linux/timer_queue.h"
+
+namespace nearby {
+namespace linux {
+
+class Timer : public api::Timer {
+ public:
+  Timer() = default;
+  ~Timer() override;
+
+  bool Create(int delay, int interval,
+              absl::AnyInvocable<void()> callback) override
+      ABSL_LOCKS_EXCLUDED(mutex_);
+  bool Stop() override ABSL_LOCKS_EXCLUDED(mutex_);
+  bool FireNow() override ABSL_LOCKS_EXCLUDED(mutex_);
+
+ private:
+  static void TimerRoutine(void *lpParam);
+
+  mutable absl::Mutex mutex_;
+  int delay_ ABSL_GUARDED_BY(mutex_);
+  int interval_ ABSL_GUARDED_BY(mutex_);
+  absl::AnyInvocable<void()> callback_;
+  uint16_t handle_ ABSL_GUARDED_BY(mutex_) = 0;
+  std::unique_ptr<TimerQueue> timer_queue_handle_;
+  std::unique_ptr<SubmittableExecutor> task_executor_ ABSL_GUARDED_BY(mutex_) =
+      nullptr;
+};
+
+}  // namespace linux
+}  // namespace nearby
+
+#endif  // PLATFORM_IMPL_LINUX_TIMER_H_