provide a way to set DEFAULT_BLOCK_SIZE_EXP and other values in Context

[ealfie]

It would be useful if there is a kind of DEFAULT_SIZE_EXP for the aiocoap.Context class, so it is easier to configure this value using inheritance.

There may be other constants, that could be useful to configure in this way.

I can make a pull request if you want.

[chrysn]

With the introduction of multi-endpoint contexts in #34, the default size should probably go somewhere in the TransportEndpoint, maybe even piggy-back with the fill_remote / have the default block size accessible via the remote. If you want to send a pull request, feel free (just post a note here if you start working on it to avoid duplicate effort).

[chrysn]

Not really a fix for the issue, but should be mentioned here anyway: In the current master, a blockwise managed application can set a Block1 option on requests or a Block2 option on a response; the block number will be discarded by the block handling mechanism, but the block size will be used for block size negotiation.

This allows at least application-level workarounds until a good solution is found (possibly bookkeeping of which endpoints negotiated which block sizes that can be pre-populated for known limited endpoints).