Combination of blockwise and observe not working

[azambranogalbis]

Observing a resource whose payload needs more than 1 block to be transferred is not working. The sequence of messages is as follows:

• Client performs first GET, with observe flag set
• Server responds back with block #0. Due to the check in https://github.com/plgd-dev/go-coap/blob/master/net/blockwise/blockwise.go#L543, this execution does not reach to store anything in sendingMessagesCache
• Client perfoms second GET of block add tcp, tcp-tls features + refactor of client, server #1
• Check at https://github.com/plgd-dev/go-coap/blob/master/net/blockwise/blockwise.go#L361, will return that sendingMessageExist is false. Final effect is that server responds block #0 again. Which breaks the logic in the client

Removing check at https://github.com/plgd-dev/go-coap/blob/master/net/blockwise/blockwise.go#L543 solves the issue.