Implement a simpler unversioned.Time serialization

Time is the number of seconds since the unix epoch and is consistent
with our JSON fields.

pkg/api/unversioned/time.go

@@ -28,8 +28,9 @@ import (
 // of the factory methods that the time package offers.
 //
 // +protobuf.options.marshal=false
+// +protobuf.as=Timestamp
 type Time struct {
-	time.Time `protobuf:"Timestamp,1,req,name=time"`
+	time.Time `protobuf:"-"`
 }
 
 // NewTime returns a wrapped instance of the provided time