Minimum Fragment MTU

[robertroeser]

I think we should include a minimum fragment size in the RSocket spec. It seems strange that the fragment size could be 1 byte - maybe there is reason I'm not aware of but it seems like should pick something and not support a fragment lower than that. Also the spec is not clear if fragmentation should include the header size as part of the fragment size - i.e. if the fragment size is 128, should it be 128+the header, or 120+8bytes for the header.