FETCH and blockwise - Behavior change of Californium 3.8

[boaks]

See the discussion in PR #2088 and Send request body when requesting Block2 messages for some more details.

The current implementation of a blockwise FETCH (bock2 only) sends the payload only in the first request. The follow up request do not contain the payload. With that, it's not possible to implement a stateless sever. Changing that, will require more data volume.

When Californium acts as server, Californium uses the client's identity (maybe the ip-address or the principal, depending on some configuration) and the URI including the URI-query to match a blockwise transfer. Since version 3.9, PR #2161 added also the message code to that. Californium's uses state for its implementation.

For a mixed block1/block2 transfer this seems to be unclear, what the best behavior will be.

If you're interested, please write us, what your preference will be.