Blockwise transfers not queued if conflicting

[chrysn]

Simultaneous blockwise (block1) requests to the same resource can not be distinguished in CoAP and thus need to be serialized or originate from different ports.

Neither happens, causing test_blockwise..test_sequential to fail.

Planned fix is the block handler noting in-progress transfers and queueing up (or maybe failing in case of NONs) the second request.

[chrysn]

As an alternative to serializing, they could also use Request-Tag.