Implement Chain Exchange protocol over pubsub

[masih]

Implement chain exchange protocol over pubsub as a mechanism to propagate ECChain across the network with reasonable spam protection.

To protect against spam the mechanism employs two separate caches for chains that are generally discovered across the network and the ones explicitly looked up or broadcasted by the local node. Both caches are capped LRU, where the LRU recent-ness is used as a way to prioritise chains we cache while keeping the total memory footprint fixed. This approach is not the most memory efficient but is simpler to implement as the LRU encapsulates a lot of the complexity.

The code has a lot of TODOs as places to improve or question to the reviewer. To action most of the TODOs further refactoring across the code is needed which is intended to be actioned in separate commits.

The code path introduced here is not integrated into F3 host; future PRs will iteratively integrate the mechanism across F3 host and other places.

Part of #792

[codecov]

Codecov Report

Attention: Patch coverage is 50.68493% with 144 lines in your changes missing coverage. Please review.

Project coverage is 68.63%. Comparing base (d4f3a0c) to head (7f3d3ae).

Files with missing lines	Patch %	Lines
chainexchange/pubsub.go	56.93%	77 Missing and 13 partials ⚠️
chainexchange/options.go	34.14%	47 Missing and 7 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #807      +/-   ##
==========================================
- Coverage   69.38%   68.63%   -0.75%     
==========================================
  Files          78       81       +3     
  Lines        7957     8249     +292     
==========================================
+ Hits         5521     5662     +141     
- Misses       1971     2101     +130     
- Partials      465      486      +21     

Files with missing lines	Coverage Δ
chainexchange/chainexchange.go	100.00% <100.00%> (ø)
internal/psutil/psutil.go	32.25% <ø> (ø)
chainexchange/options.go	34.14% <34.14%> (ø)
chainexchange/pubsub.go	56.93% <56.93%> (ø)

... and 3 files with indirect coverage changes