BUG-5280: optimize identifier serialization format

BUG-5280: optimize identifier serialization format

Using ObjectOutput.writeObject() results in large overhead, resulting
in transaction identifiers weighing in at 549 bytes when used in an object.

Introduce WritableObject concept, which has a WriteTo(ObjectOutput) method
and a static factory method readForm(ObjectInput). This effectively means
we do not get Java Serialization headers.

Also make FrontendType a final class, adopting the WritableObject serialization,
which brings down the overhead to below 100 bytes.

Change-Id: I20294d4fdf309f250d507dfc675d8405c1fcf505
Signed-off-by: Robert Varga <rovarga@cisco.com>