ReentrantLock get(QuotaResource quotaResource) {
      return mutexCache.computeIfAbsent(quotaResource, k -> new ReentrantLock());
    }
Returns the mutex in the form of {@link ReentrantLock} object for the specified {@link QuotaResource}.
If the mutex is not already present in the cache, it creates a new mapping.
@param quotaResource {@link QuotaResource} object.
@return ReentrantLock object as mutex for the specified quotaResource.
ambry-quota/src/main/java/com/github/ambry/quota/QuotaResourceSynchronizer.java