use the linux monotonic clock instead of gettimeofday

luprex/core/cpp/util.hpp

@@ -37,8 +37,8 @@ using LuaSourcePtr = std::unique_ptr<LuaSourceVec>;
 using HashValue = std::pair<uint64_t, uint64_t>;
 using IdVector = std::vector<int64_t>;
 
-// Return nanoseconds elapsed, for profiling purposes.
-int64_t profiling_clock();
+// Return seconds elapsed, for profiling purposes.
+double profiling_clock();
 
 // Return true if the string is a valid lua identifier.
 bool is_identifier(const std::string &str);