Switch the value type for the configuration holder class from Long to
String. Any method that expects the configuration value to be Long will
now parseLong the String, or return a default value, defined in the
code.
Add a custom account number generator that takes in a format string and
generates account numbers based upon it.
Refactor AccountNumberGeneratorFactory and AccountNumberGenerator.

Add more configuration options to the Configuration Domain Service for
the account format field.

Also effect changes across the board wherever it is used.