Create an internal magic number (i.e. -1 or 255) for user defined externalizers. This is done to avoid users using our number space.

So, internally: <magic_number> <stream>
Users: <magic_number><user defined magic number (int)> <stream>

Mandate unsigned ints so that we can optimise by sending them as variable lenght
Internal frameworks could use high enough numbers for example up to 2 bytes: 5000, 7000, 20000
1 byte: 128
2 bytes: 32767
3 bytes: ...

GlobalConfiguration.registerMarshallable(Class type, Externalizer ext, int id);

Maybe CacheManager better?

CacheManager.registerMarshallable(Class type, Externalizer ext, int id);

Future improvement, maybe generate ids automatically for user defined classes?