Use steady_clock to generate the seed for srand

And remove outdates comments.

srtcore/api.cpp

@@ -194,14 +194,8 @@ m_bGCStatus(false),
 m_ClosedSockets()
 {
    // Socket ID MUST start from a random value
-   // Note. Don't use CTimer here, because s_UDTUnited is a static instance of CUDTUnited
-   // with dynamic initialization (calling this constructor), while CTimer has
-   // a static member s_ullCPUFrequency with dynamic initialization.
-   // The order of initialization is not guaranteed.
-   timeval t;
-
-   gettimeofday(&t, 0);
-   srand((unsigned int)t.tv_usec);
+   unsigned seed = count_microseconds(steady_clock::now().time_since_epoch()) & UINT32_MAX;
+   srand(seed);
 
    const double rand1_0 = double(rand())/RAND_MAX;