[Roadmap issue] Short-term hardening

[raulk]

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

• Expose the peer that propagated the message to us (Expose the peer that propagates a message to the recipient #218)
• Subscription options, which enables...
• Topic handles (Topic handles and ownership #198), so we can...
• Configurable per-topic max message sizes (Allow per-topic message size limits #209).
• [ ] Duplicate consumption (Have a configuration option for Subscriptions receiving duplicate messages #216), which is blocked by...
• [ ] Validator cache (we don’t want to validate dupes over and over again, requires message signing to be enabled).
• Configurable outbound peer queue sizes (Published messages can be dropped silently #217).
• Make Publish() report problems / coalesce queued messages (Published messages can be dropped silently #217).
• Mark topics as relay-only (Subscribe and Relay should be separate operations. #28).
• Relative prioritization of traffic per topic (needs to be designed and thought carefully) (Allow prioritization of traffic per topic #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.