L2: drip rewards from L1, introducing Reservoirs

[pcarranzav]

The changes from this PR correspond to GIP-0034

This is a rebased version of #571, cleaned up to only include the changes for the reservoir drip rewards distribution mechanism. Rebased on top of #699 which includes the bridge with all the audit fixes.

[codecov]

Codecov Report

Base: 91.53% // Head: 92.34% // Increases project coverage by +0.80% 🎉

Coverage data is based on head (648f54d) compared to base (8aa3bd6).
Patch coverage: 99.48% of modified lines in pull request are covered.

❗ Current head 648f54d differs from pull request most recent head 559ea00. Consider uploading reports for the commit 559ea00 to get more accurate results

Additional details and impacted files

@@                Coverage Diff                @@
##           pcv/l2-bridge     #701      +/-   ##
=================================================
+ Coverage          91.53%   92.34%   +0.80%     
=================================================
  Files                 42       44       +2     
  Lines               1984     2141     +157     
  Branches             346      374      +28     
=================================================
+ Hits                1816     1977     +161     
+ Misses               168      164       -4     

Flag	Coverage Δ
unittests	92.34% <99.48%> (+0.80%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
contracts/tests/ReservoirMock.sol	66.66% <66.66%> (ø)
contracts/governance/Managed.sol	100.00% <100.00%> (+5.45%)	⬆️
contracts/l2/reservoir/L2Reservoir.sol	100.00% <100.00%> (ø)
contracts/reservoir/L1Reservoir.sol	100.00% <100.00%> (ø)
contracts/reservoir/Reservoir.sol	100.00% <100.00%> (ø)
contracts/rewards/RewardsManager.sol	100.00% <100.00%> (ø)
contracts/staking/Staking.sol	96.77% <100.00%> (+0.48%)	⬆️
contracts/l2/gateway/L2GraphTokenGateway.sol	100.00% <0.00%> (ø)
contracts/tests/CallhookReceiverMock.sol

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[pcarranzav]

Closing as GIP-0037 is believed to be the better alternative at this point (see https://forum.thegraph.com/t/gip-0037-the-graph-arbitrum-deployment-with-linear-rewards-minted-in-l2/3551)