Store the count of unique Receieved cids

[aarshkshah1992]

This PR introduces the minimal set of changes we need to fix the logging hotspot in Lotus that happens because of reading the entire list of received cids from disk.

Lotus PR at:
filecoin-project/lotus#6385

@dirkmc I've made this PR against master but we need to merge it into v1.6.0 as well and release to avoid the latest changes in master from making their way to these fixes, right ?

[codecov-commenter]

Codecov Report

Merging #216 (8f4ef36) into master (db84ad0) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #216      +/-   ##
==========================================
+ Coverage   64.84%   64.88%   +0.04%     
==========================================
  Files          24       24              
  Lines        2600     2603       +3     
==========================================
+ Hits         1686     1689       +3     
  Misses        573      573              
  Partials      341      341              

Impacted Files	Coverage Δ
channels/channel_state.go	81.08% <100.00%> (+0.52%)	⬆️
channels/channels_fsm.go	63.93% <100.00%> (+0.29%)	⬆️

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[dirkmc]

Closing in favour of #217