Evaluate usability of BERT as a model for blockwise interaction

[chrysn]

The current code for handling blockwise is relatively complex, is not fully satisfying (#44, #45) and includes library-specific workarounds.

Modelling the process after the BERTs from draft-ietf-core-coap-tcp-tls-05 could clarify things -- a program would ask the library to exchange "jumbo" messages (even without explicit BERT flagging) that are always final (0, 0, 7). The library would than act exactly like a proxy towards the application (unless "hands off blocks" is requested).

[chrysn]

This could be trickier as szx=7 only means BERT when the transport wants it to, while aiocoap handles messages in a mostly transport agnostic way.

Possibly this will be an issue with sufficiently many implementations that no practical other meaning for szx=7 can be established.