Mempool: reduce size of sigValCache

[arajasek]

No description provided.

[codecov]

Codecov Report

Merging #7305 (aef741a) into master (d1d8845) will decrease coverage by 0.00%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #7305      +/-   ##
==========================================
- Coverage   39.12%   39.11%   -0.01%     
==========================================
  Files         614      614              
  Lines       64983    64984       +1     
==========================================
- Hits        25422    25421       -1     
+ Misses      35155    35152       -3     
- Partials     4406     4411       +5     

Impacted Files	Coverage Δ
chain/messagepool/messagepool.go	56.93% <50.00%> (-0.20%)	⬇️
chain/events/message_cache.go	87.50% <0.00%> (-12.50%)	⬇️
chain/exchange/peer_tracker.go	66.66% <0.00%> (-4.31%)	⬇️
chain/sub/incoming.go	54.43% <0.00%> (-3.38%)	⬇️
storage/wdpost_sched.go	77.22% <0.00%> (-1.99%)	⬇️
chain/events/events_called.go	83.90% <0.00%> (-1.96%)	⬇️
storage/wdpost_changehandler.go	97.64% <0.00%> (-0.95%)	⬇️
extern/sector-storage/sched_worker.go	77.25% <0.00%> (-0.73%)	⬇️
chain/messagepool/selection.go	80.12% <0.00%> (-0.41%)	⬇️
miner/miner.go	52.98% <0.00%> (-0.34%)	⬇️
... and 8 more

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