Time: Implement duration_cast from DeltaTimeval to std::chrono::duration

This will allow us to migrate step-by-step from DeltaTimeval to std::chrono.

src/zm_time.h

@@ -247,7 +247,28 @@ struct posix_duration_cast<timeval, std::chrono::duration<Rep, Period>> {
   }
 };
 
+// chrono -> DeltaTimeval caster
+template<typename Rep, typename Period>
+struct posix_duration_cast<std::chrono::duration<Rep, Period>, DeltaTimeval> {
+  template<uint32 Prec>
+  static DeltaTimeval cast(std::chrono::duration<Rep, Period> const &d) {
+    typedef std::chrono::duration<int64, std::ratio<1, Prec>> fsec_t;
+    DeltaTimeval res = {};
+
+    Seconds secs = std::chrono::duration_cast<Seconds>(d);
+    fsec_t fsec = std::chrono::duration_cast<fsec_t, int64, std::ratio<1, Prec>>(d - secs);
+
+    res.positive = fsec >= Seconds::zero();
+    res.delta = abs((secs + fsec).count());
+    res.sec = secs.count();
+    res.fsec = fsec.count();
+    res.prec = Prec;
+
+    return res;
+  }
+};
 }
+
 // chrono -> timeval
 template<typename T, typename Rep, typename Period>
 auto duration_cast(std::chrono::duration<Rep, Period> const &d)
@@ -260,6 +281,13 @@ template<typename Duration>
 Duration duration_cast(timeval const &tv) {
   return impl::posix_duration_cast<timeval, Duration>::cast(tv);
 }
+
+// chrono -> DeltaTimeval
+template<typename T, uint32 Prec, typename Rep, typename Period>
+auto duration_cast(std::chrono::duration<Rep, Period> const &d)
+-> typename std::enable_if<std::is_same<T, DeltaTimeval>::value, DeltaTimeval>::type {
+  return impl::posix_duration_cast<std::chrono::duration<Rep, Period>, DeltaTimeval>::template cast<Prec>(d);
+}
 }
 }