Updatable epoch length / seat parameters

[ilblackdragon]

Currently epoch length and seat parameters (number of block validators, fisheman in shards) are constants in genesis.
Generally validator_manager supports their change, but it must be recorded somewhere in the headers and updated as we catching up with the network.

[stale]

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[bowenwang1996]

Relevant discussion https://gov.near.org/t/store-protocol-config-in-a-contract/2105