Added monotonic_clock class in time.h

Change-Id: Iea80a7cde09a985f7fbf64090c58aeccde38e949
diff --git a/aos/common/time.h b/aos/common/time.h
index a176ba0..40b72b0 100644
--- a/aos/common/time.h
+++ b/aos/common/time.h
@@ -7,12 +7,31 @@
 #include <stdint.h>
 
 #include <type_traits>
+#include <chrono>
+#include <thread>
 #include <ostream>
 
 #include "aos/common/type_traits.h"
 #include "aos/common/macros.h"
 
 namespace aos {
+
+class monotonic_clock {
+ public:
+  typedef ::std::chrono::nanoseconds::rep rep;
+  typedef ::std::chrono::nanoseconds::period period;
+  typedef ::std::chrono::nanoseconds duration;
+  typedef ::std::chrono::time_point<monotonic_clock> time_point;
+
+  static monotonic_clock::time_point now() noexcept;
+  static constexpr bool is_steady = true;
+
+  // Returns the epoch (0).
+  static constexpr monotonic_clock::time_point epoch() {
+    return time_point(duration(0));
+  }
+};
+
 namespace time {
 
 // A nice structure for representing times.
@@ -280,4 +299,15 @@
 }  // namespace time
 }  // namespace aos
 
+namespace std {
+namespace this_thread {
+// Template specialization for monotonic_clock, since we can use clock_nanosleep
+// with TIMER_ABSTIME and get very precise absolute time sleeps.
+template <>
+void sleep_until(const ::aos::monotonic_clock::time_point &end_time);
+
+}  // namespace this_thread
+}  // namespace std
+
+
 #endif  // AOS_COMMON_TIME_H_