static void subtract(Map<String, Map<String, Long>> storageUsage, Map<String, Map<String, Long>> base) {
    for (Map.Entry<String, Map<String, Long>> accountStorageUsageEntry : storageUsage.entrySet()) {
      String accountId = accountStorageUsageEntry.getKey();
      if (base.containsKey(accountId)) {
        Map<String, Long> containerUsage = accountStorageUsageEntry.getValue();
        Map<String, Long> baseContainerUsage = base.get(accountId);
        containerUsage.replaceAll((k, v) -> Math.max(v - baseContainerUsage.getOrDefault(k, (long) 0), 0));
      }
    }
  }
Subtract values of {@code base} from values of {@code containerStorageUsage}.
If values exist in {@code containerStorageUsage} but not in {@code base}, then nothing happens.
If values exist in {@code base} but not in {@code containerStorageUsage}, then nothing happens either.
If values in {@code containerStorageUsage} are less than values in {@code base}, then the result is 0.
@param storageUsage
@param base
ambry-quota/src/main/java/com/github/ambry/quota/storage/MySqlStorageUsageRefresher.java