Check stale validators with RPC

[lukaw3d]

Resolves #751

Summary of new flow to fetch validators from comments:

• Skip if validators are already in store, and network didn't change
• Fetch validators from API
• If API fails on testnet/local, fall back to empty list
• If fetching dump_validators fails, fall back to empty list
• If API fails on mainnet, fall back to dump_validators, and refresh validators' status with RPC
• If RPC fails, fall back to dump_validators with unknown validators' status
• Fall back to dump_validators with refreshed validators' status from RPC

[github-actions]

MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ EDITORCONFIG	editorconfig-checker	13		0	0.57s
✅ GIT	git_diff	yes		no	0.04s
✅ JSON	eslint-plugin-jsonc	2	0	0	1.1s
✅ JSON	jsonlint	2		0	0.56s
✅ JSON	prettier	2	0	0	0.74s
✅ JSON	v8r	2		0	1.93s
✅ TSX	eslint	2	0	0	6.1s
✅ TYPESCRIPT	eslint	6	0	0	5.45s

See errors details in artifact MegaLinter reports on CI Job page
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[codecov]

Codecov Report

Merging #776 (c745647) into master (09cbc04) will decrease coverage by 0.24%.
The diff coverage is 79.16%.

@@            Coverage Diff             @@
##           master     #776      +/-   ##
==========================================
- Coverage   88.52%   88.28%   -0.25%     
==========================================
  Files         100      100              
  Lines        1664     1698      +34     
  Branches      332      339       +7     
==========================================
+ Hits         1473     1499      +26     
- Misses        191      199       +8     

Flag	Coverage Δ
cypress	58.01% <31.25%> (-0.86%)	⬇️
jest	74.89% <75.00%> (-0.09%)	⬇️

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Impacted Files	Coverage Δ
...pages/StakingPage/Features/ValidatorList/index.tsx	74.28% <0.00%> (-13.60%)	⬇️
src/app/state/staking/index.ts	89.28% <40.00%> (-3.82%)	⬇️
src/app/state/staking/saga.ts	94.89% <86.48%> (-0.63%)	⬇️
src/app/state/staking/selectors.ts	100.00% <100.00%> (ø)

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[buberdds]

When it can be undefined? Do we want to show 1970 in updateValidatorsError?

[lukaw3d]

1970 should never appear with current code. It is undefined until first maybeRefreshValidators (initialState = { validators: null })

Annoying options:

1. keep default 0, so type is number
2. default to null, and force checking nullable everywhere we want to display timestamp. You have to check nullable even if we already check updateValidatorsError.
3. create selector for state.validators, so that it is a single variable for nullable checks. But it probably isn't best practice, not minimal for mocking, and redux wouldn't detect changes if subfield changed.

[lukaw3d]

Hm, wait, 2. is fine if we just use non-null assertion