[cip-56] Remove freezable check on epoch rewards

[gastonponti]

Description

Implementation of the CIP-56 (https://github.com/celo-org/celo-proposals/blob/master/CIPs/cip-0056.md)

Tested

Run tests

Backwards compatibility

Part of the Gingerbread Hard Fork

[github-actions[bot]]

Coverage from tests in ./e2e_test/... for ./consensus/istanbul/... at commit f1ecb60f865e1267bd39cac2488bcf185fd22f16

coverage: 49.7% of statements across all listed packages

coverage:  63.2% of statements in consensus/istanbul
coverage:  43.1% of statements in consensus/istanbul/announce
coverage:  55.7% of statements in consensus/istanbul/backend
coverage:   0.0% of statements in consensus/istanbul/backend/backendtest
coverage:  24.3% of statements in consensus/istanbul/backend/internal/replica
coverage:  60.9% of statements in consensus/istanbul/core
coverage:  50.0% of statements in consensus/istanbul/db
coverage:   0.0% of statements in consensus/istanbul/proxy
coverage:  64.4% of statements in consensus/istanbul/uptime
coverage:  51.8% of statements in consensus/istanbul/validator
coverage:  79.2% of statements in consensus/istanbul/validator/random

[github-actions[bot]]

[codecov[bot]]

Codecov Report

Patch coverage has no change and project coverage change: +0.10% :tada:

Comparison is base (4a1e3fc) 55.16% compared to head (04d481e) 55.27%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2164 +/- ## ========================================== + Coverage 55.16% 55.27% +0.10% ========================================== Files 676 676 Lines 113990 113991 +1 ========================================== + Hits 62880 63005 +125 + Misses 47229 47124 -105 + Partials 3881 3862 -19 ``` | [Files Changed](https://app.codecov.io/gh/celo-org/celo-blockchain/pull/2164?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=celo-org) | Coverage Δ | | |---|---|---| | [consensus/istanbul/backend/pos.go](https://app.codecov.io/gh/celo-org/celo-blockchain/pull/2164?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=celo-org#diff-Y29uc2Vuc3VzL2lzdGFuYnVsL2JhY2tlbmQvcG9zLmdv) | `5.74% <0.00%> (-1.77%)` | :arrow_down: | ... and [31 files with indirect coverage changes](https://app.codecov.io/gh/celo-org/celo-blockchain/pull/2164/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=celo-org)

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.