[Roadmap issue] Short-term hardening

[raulk]

Features needed by Filecoin and ETH2, in order of priority:

• [X] Expose the peer that propagated the message to us (#218)
• [X] Subscription options, which enables...
• [x] Topic handles (#198), so we can...
• [ ] Configurable per-topic max message sizes (#209).
• ~[ ] Duplicate consumption (#216), which is blocked by...~
• ~[ ] Validator cache (we don’t want to validate dupes over and over again, requires message signing to be enabled).~
• [x] Configurable outbound peer queue sizes (#217).
• [ ] Make Publish() report problems / coalesce queued messages (#217).
• [ ] Mark topics as relay-only (#28).
• [ ] Relative prioritization of traffic per topic (needs to be designed and thought carefully) (#210).

[vyzo]

Duplicate consumption can be implemented by the application through a custom tracer, just like the score subsystem plugs into the tracer. The validator cache, which is complex, is then unecessary.