[Committee] Fallback mechanism for DecodeSignerIDs

[durkmurder]

This PR attempts to fix error which happens on mainnet:

$ flow blocks get 47169688 -n mainnet 
:x: Command Error: error fetching block: client: rpc error: code = Unknown desc = could not retrieve identities for block 424d4016debbab47e5feeeda3f63957d8f559d413dae28510cb7950a1ea53b46 with QC view 0: by-view query for unknown epoch

When requesting blocks from old epoch(not previous or current) we end up calling IdentitiesByEpoch which relies on local cache, and this cache doesn't have that epoch. This is the reason why getting an old block fails.
Proposed hot-fix is to use IdentitiesByBlock as fallback if the primary path has failed.

[codecov-commenter]

Codecov Report

Merging #4063 (39bb40e) into v0.29 (757143d) will decrease coverage by 3.16%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##            v0.29    #4063      +/-   ##
==========================================
- Coverage   52.40%   49.25%   -3.16%     
==========================================
  Files         757      531     -226     
  Lines       71468    50059   -21409     
==========================================
- Hits        37455    24657   -12798     
+ Misses      30919    23277    -7642     
+ Partials     3094     2125     -969     

Flag	Coverage Δ
unittests	49.25% <ø> (-3.16%)	⬇️

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see 229 files with indirect coverage changes

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[peterargue]

thanks @durkmurder!