[Epoch Sync] Support bootstrapping a node from another epoch-synced node.

[robin-near]

• Store EpochSyncProof locally after as part of bootstrapping the node.
• Use the stored EpochSyncProof to derive a new EpochSyncProof in order to handle EpochSyncRequests.
• Add a new testloop test to check that one can bootstrap a new node via epoch sync from a node that was bootstrapped with epoch sync itself
• Add new testloop tests to check that the EpochSyncProof derived based on an older EpochSyncProof is exactly equal to an EpochSync derived from genesis.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 77.35849% with 12 lines in your changes missing coverage. Please review.

Project coverage is 71.49%. Comparing base (cdb8055) to head (0d7b270). Report is 16 commits behind head on master.

Files with missing lines	Patch %	Lines
chain/client/src/sync/epoch.rs	74.46%	8 Missing and 4 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #12100 +/- ## ========================================== - Coverage 71.61% 71.49% -0.12% ========================================== Files 818 819 +1 Lines 164524 164896 +372 Branches 164524 164896 +372 ========================================== + Hits 117816 117900 +84 - Misses 41569 41840 +271 - Partials 5139 5156 +17 ``` | [Flag](https://app.codecov.io/gh/near/nearcore/pull/12100/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=near) | Coverage Δ | | |---|---|---| | [backward-compatibility](https://app.codecov.io/gh/near/nearcore/pull/12100/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=near) | `0.17% <0.00%> (-0.01%)` | :arrow_down: | | [db-migration](https://app.codecov.io/gh/near/nearcore/pull/12100/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=near) | `0.17% <0.00%> (-0.01%)` | :arrow_down: | | [genesis-check](https://app.codecov.io/gh/near/nearcore/pull/12100/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=near) | `1.26% <0.00%> (-0.01%)` | :arrow_down: | | [integration-tests](https://app.codecov.io/gh/near/nearcore/pull/12100/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=near) | `38.63% <75.47%> (-0.07%)` | :arrow_down: | | [linux](https://app.codecov.io/gh/near/nearcore/pull/12100/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=near) | `71.29% <77.35%> (+0.10%)` | :arrow_up: | | [linux-nightly](https://app.codecov.io/gh/near/nearcore/pull/12100/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=near) | `71.09% <77.35%> (-0.04%)` | :arrow_down: | | [macos](https://app.codecov.io/gh/near/nearcore/pull/12100/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=near) | `53.50% <4.08%> (-0.57%)` | :arrow_down: | | [pytests](https://app.codecov.io/gh/near/nearcore/pull/12100/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=near) | `1.52% <0.00%> (-0.01%)` | :arrow_down: | | [sanity-checks](https://app.codecov.io/gh/near/nearcore/pull/12100/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=near) | `1.33% <0.00%> (-0.01%)` | :arrow_down: | | [unittests](https://app.codecov.io/gh/near/nearcore/pull/12100/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=near) | `65.22% <4.08%> (-0.18%)` | :arrow_down: | | [upgradability](https://app.codecov.io/gh/near/nearcore/pull/12100/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=near) | `0.21% <0.00%> (-0.01%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=near#carryforward-flags-in-the-pull-request-comment) to find out more.

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