Support specifying a preferred block size for server responses

[AlCalzone]

As defined in https://tools.ietf.org/html/rfc7959#section-2.4, clients can specify a preferred block size for the responses from the CoAP server by using the Block2 option on a request.

This size should be configurable in the library. When used with block-wise requests, the Block2 option should only appear in the last fragment, as defined in https://tools.ietf.org/html/rfc7959#section-2.7

• [x] Specify blocksize
• [ ] Support combination with block-wise requests

[AlCalzone]

1st bullet point implemented in 2cf786d