Add a threshold above which rewards start accruing for a subgraph

[abarmat]

Summary

Add a threshold of token signaled that a subgraph needs to have as a minimum to start accruing rewards.

[codecov]

Codecov Report

Merging #528 (7f427b2) into dev (2cf753f) will decrease coverage by 0.22%.
The diff coverage is 86.66%.

@@            Coverage Diff             @@
##              dev     #528      +/-   ##
==========================================
- Coverage   90.60%   90.37%   -0.23%     
==========================================
  Files          35       37       +2     
  Lines        1703     1798      +95     
  Branches      282      294      +12     
==========================================
+ Hits         1543     1625      +82     
- Misses        160      173      +13     

Flag	Coverage Δ
unittests	90.37% <86.66%> (-0.23%)	⬇️

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Impacted Files	Coverage Δ
contracts/discovery/GNS.sol	88.57% <ø> (+1.45%)	⬆️
contracts/rewards/RewardsManager.sol	95.95% <20.00%> (-4.05%)	⬇️
contracts/curation/Curation.sol	100.00% <100.00%> (ø)
contracts/curation/GraphCurationToken.sol	100.00% <100.00%> (ø)
contracts/staking/Staking.sol	96.26% <0.00%> (-2.74%)	⬇️
contracts/base/SubgraphNFT.sol
contracts/base/SubgraphNFTDescriptor.sol
contracts/discovery/SubgraphNFTDescriptor.sol	100.00% <0.00%> (ø)
... and 4 more

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