This is a simple implementation of the TypeDatabase interface that
provides persistent storage for service types.  All service types
are stored in a single file "types.dat".

Serializable objects are used to maintain the persistent state of
the service type database.  The Type, TypeProperty and Incarnation
classes are serializable.  As such, they each have serialVersionUID
variables defined, which MUST be updated if the persistent representation
of the class changes (i.e. member data is added or removed).

Note that once the service types have been loaded from the file,
they are cached in memory from that point on.
