Centralize logic to get current server time

A few different implementations of computing the current time were
spread out through the code base, most of them using gettimeofday().

This centralizes the logic in CUtil::GetTime() for easier maintenance,
and also allows all call sites to get the benefit of the clock_gettime()
code path on systems that support it.

test/UtilsTest.cpp

@@ -95,11 +95,7 @@ TEST(UtilsTest, ServerTime) {
     CString str1 = CUtils::FormatServerTime(tv1);
     EXPECT_EQ("2011-10-19T16:40:51.620Z", str1);
 
-    timeval now;
-    if (gettimeofday(&now, nullptr)) {
-        now.tv_sec = time(nullptr);
-        now.tv_usec = 0;
-    }
+    timeval now = CUtils::GetTime();
 
     // Strip microseconds, server time is ms only
     now.tv_usec = (now.tv_usec / 1000) * 1000;