Use omrtime_hires_clock() for monotonic time when necessary in gc

The omrtime_current_time_millis() uses UNIX gettimeofday() function,
and is therefore non-monotonic and error-prone.
We turn to use omrtime_hires_clock() function call
whenever we need the time to be monotonic.

Issue: #7051

gc/base/standard/Scavenger.cpp

@@ -3288,7 +3288,7 @@ MM_Scavenger::getFreeCache(MM_EnvironmentStandard *env)
  if (NULL == cache) {
  env->_scavengerStats._scanCacheOverflow = 1;
  OMRPORT_ACCESS_FROM_OMRPORT(env->getPortLibrary());
- uint64_t duration = omrtime_current_time_millis();
+ uint64_t duration = omrtime_hires_clock();
 
  bool resizePerformed = false;
  omrthread_monitor_enter(_freeCacheMonitor);
@@ -3305,8 +3305,8 @@ MM_Scavenger::getFreeCache(MM_EnvironmentStandard *env)
  /* Still need a new cache and nothing left reserved - create it in Heap */
  cache = createCacheInHeap(env);
  }
- duration = omrtime_current_time_millis() - duration;
- env->_scavengerStats._scanCacheAllocationDurationDuringSavenger += duration;
+ duration = omrtime_hires_clock() - duration;
+ env->_scavengerStats._scanCacheAllocationDurationDuringSavenger += omrtime_hires_delta(0, duration, OMRPORT_TIME_DELTA_IN_MILLISECONDS);
 
  }