Add the ability to listen for discovered chains

[masih]

Expand chain exchange to accept a listener which is notified whenever a new chain is discovered. This mechanism is intended to be integrated into F3 host pubsub, whereupon receiving a partial message the host looks up its chain. When known, the chain is returned immediately. Otherwise, the host would buffer the partial message and await notification of its discovering from chain exchange.

Part of #792

[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 10 lines in your changes missing coverage. Please review.

Project coverage is 68.54%. Comparing base (135bfe6) to head (7c4840e).

Files with missing lines	Patch %	Lines
chainexchange/pubsub.go	75.00%	7 Missing ⚠️
chainexchange/options.go	57.14%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #808      +/-   ##
==========================================
- Coverage   68.97%   68.54%   -0.44%     
==========================================
  Files          81       81              
  Lines        8255     8281      +26     
==========================================
- Hits         5694     5676      -18     
- Misses       2079     2116      +37     
- Partials      482      489       +7     

Files with missing lines	Coverage Δ
chainexchange/chainexchange.go	100.00% <ø> (ø)
chainexchange/options.go	35.95% <57.14%> (+1.80%)	⬆️
chainexchange/pubsub.go	55.90% <75.00%> (-0.31%)	⬇️

... and 5 files with indirect coverage changes

[Kubuxu]

If you are saying we are fine with doing it under lock, we should keep it under lock. Manual interrupted locking like this is a foot gun.

[masih]

Manual interrupted locking like this is a foot gun.

I take this as a blocker? It's either this or notify listener under lock. I have no preference.

Taking a closer look at the lru library we are using, I am sort of underwhelmed that it doesn't expose atomic combined removal operations. if we had those, we could remove the need for lock in most places. I also understand why they haven't , i.e. the auto eviction of lru used for caching.