Apply consensus committee changes at epoch boundaries

[JamesHinshelwood]

The most important changes in this commit are in deposit.sol. Everything else is pretty much just supporting the new interfaces in the deposit contract. The key feature is that we now keep track of the current 'epoch' and only apply changes to the consensus committee two epochs after they are made.

There are a few other changes to the deposit contract in this commit too:

• The initial stakers are now passed to the contract constructor, rather than relying on the setStake hack.
• leader() was removed since it isn't used anywhere and it doesn't make sense to have both leader and leaderAtView.
• Delayed withdrawals are now supported - We keep a circular buffer of pending withdrawals for each validator. 2 weeks after unstaking, the validator can make another call to withdraw the funds from the deposit contract.

Reworked version of #1298.

Further work out of scope of this PR:

• Fill out missing integration tests - Additional integration tests for staking #1341
• Implement unsuspend, and consensus-based suspend - Staking: implement suspension features #1337
• Epoch length tracking in EVM - part of Move consensus config on-chain (into shard contract) #1135. (Currently, the epoch length is hardcoded in deposit.sol.)
• Improving staker address and pubkey validation on registration - Improve validator address verification in deposit contract #1297
• Allowing validators to change their registered addresses - Control and reward address changes for validators #1301
• Adjust slashing and suspension queues - Staking: fix total_stake change on slashing #1338
• Consider implementing churn limits on the validator set - Queue/churn limits in deposit.sol #1300
• Improving epoch change transaction tracking - Gas (and account) tracking for epoch changes #1299
• Slashing mechanisms - currently, this reuses the previous mechanism where any validator with >=10% of total stake can slash any other validator. Additionally, slashed funds are not kept track of (effectively, burned).
• Allow reusing unstaked-but-not-yet-withdrawn funds - Deposit: allow reusing unstaked balance #1329
• Not directly necessary, but somewhat related: block number tracking - Review block-related constants passed to the EVM; potentially fix epoch ticking #1339

[86667]

Nice! Couple minor comments/questions