feat(lib/grandpa): update grandpa catch-up process; add submodule, use NeighbourMessages

[noot]

Changes

• add catchUp submodule to track status of catch up and deal with sending out catch-up requests and handling responses
• update grandpa message handler to use NeighbourMessages to trigger catch-up process if needed
• TODO: still need to test this w/ polkadot (seems we aren't getting responses, probably continue this is another PR)
• TODO: need to add test cases and update stress test

Tests

go test ./lib/grandpa -short

Issues

• closes lib/grandpa: ensure catch-up logic works when interacting with substrate nodes #1531

[codecov]

Codecov Report

Merging #1554 (a190d03) into development (2f9f80c) will decrease coverage by 1.67%.
The diff coverage is 33.71%.

❗ Current head a190d03 differs from pull request most recent head 550a625. Consider uploading reports for the commit 550a625 to get more accurate results

@@               Coverage Diff               @@
##           development    #1554      +/-   ##
===============================================
- Coverage        60.62%   58.94%   -1.68%     
===============================================
  Files              202      185      -17     
  Lines            27286    19404    -7882     
===============================================
- Hits             16541    11438    -5103     
+ Misses            8845     5980    -2865     
- Partials          1900     1986      +86     

Flag	Coverage Δ
unit-tests	58.94% <33.71%> (-1.68%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
dot/network/service.go	69.12% <0.00%> (+11.15%)	⬆️
lib/grandpa/network.go	52.03% <0.00%> (+5.26%)	⬆️
lib/grandpa/catch_up.go	31.06% <31.06%> (ø)
dot/network/notifications.go	65.47% <37.50%> (-0.38%)	⬇️
lib/grandpa/message_handler.go	69.37% <61.53%> (+4.94%)	⬆️
lib/grandpa/grandpa.go	59.87% <100.00%> (+1.38%)	⬆️
dot/network/gossip.go	37.50% <0.00%> (-62.50%)	⬇️
dot/network/pool.go	50.00% <0.00%> (-50.00%)	⬇️
dot/state/transaction.go	58.33% <0.00%> (-21.36%)	⬇️
dot/types/consensus_digest.go	0.00% <0.00%> (-17.86%)	⬇️
... and 228 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2f9f80c...550a625. Read the comment docs.

[danforbes]

It seems that the GRANDPA catch-up mechanism was implemented before request/response protocols were defined. I believe that these messages should be sent as "request notifications" and "response notifications".