Add a converter for CurrencyUnits stored in the database (#334)

* Add a converter for CurrencyUnits stored in the database

This uses the well-known String representation for currency units. It also
provides a base class for other converters that will be persisting the
toString() representation.

* Add DB and formatting changes

* Add tests, make minor fixes

util/src/main/java/google/registry/util/Retrier.java

@@ -146,9 +146,7 @@ public class Retrier implements Serializable {
   }
 
   private <V> V callWithRetry(
-      Callable<V> callable,
-      FailureReporter failureReporter,
-      Predicate<Throwable> isRetryable) {
+      Callable<V> callable, FailureReporter failureReporter, Predicate<Throwable> isRetryable) {
     int failures = 0;
     while (true) {
       try {