public ConcurrentMap<QuotaResource, ReentrantLock> getMutexMap() {
      return quotaResourceMutexCache.mutexCache;
    }
Method to return {@link QuotaResourceSynchronizer}'s mutex cache for tests.
@return ConcurrentMap of {@link QuotaResource} id and its mutex {@link ReentrantLock}.
ambry-quota/src/test/java/com/github/ambry/quota/QuotaResourceSynchronizerTest.java