Differing implementation of gossipsub that handles errors and uses hashes of messages and topics instead of IDs

[jamesray1]

This is a partial implementation of gossipsub that differs to the spec by handling errors and using hashes of messages and topics instead of IDs. While it is possible to use an enum containing either a hash or an ID, after trying to implement that I stopped due to additonal complexities around doing so, while there does not seem to be any advantage to using an ID over a hash.

Handled errors include when a topic is not in the mesh, and the local peer is not connected to the remote peer.
Relevant lines in the spec include:

• https://github.com/libp2p/rust-libp2p/pull/767/files#diff-c0f248eeadd1b209ed0f7a48476b360fR44-76
• Also CTRL+F graft_errs and prune_errs in layer.

Related: #130, #116

[raulk]

Thanks for the info, but this is not actionable. Implementors are welcome to submit RFCs to evolve the spec when they find areas for improvement.