Update Locking to handle upcoming block.time change

[baroooo]

Description

With Celo transitioning to a L2, the block time will be reduced from 5 seconds to 1 second. This change impacts our Locking and Governance mechanisms, which currently depend on a block time of 5 seconds, calculating a "week" as 120960 blocks. With the new block time, a week will be equal 604800 blocks.

To ensure the continuity of functionality after the L2 upgrade, we need to update the WEEK constant and adjust startingPointWeek and roundTimestamp such that the week calculation in the Locking contract remains unchanged post-upgrade.

• Add a NEW_WEEK constant in the contract to accommodate the new block time (1s/block) 604,800 blocks per week.
• Create variables: mentoLabsMultisig address, l2Block(uint256), newStartingWeek(uint256), oldEpochShift(constant uint32), newEpochShift(uint32) and paused(bool)
• Setters for these vars, should be only callable by the mentoLabsMultisig
• setL2TransitionBlock, which allows us to specify the exact block number for the L2 transition once it is known, will:
  set l2Block no
  pause the contract
• roundTimestamp should be pausable, and it should pause all functionality after the l2Block is set
• roundTimestamp should use the correct WEEK, startingPointWeek, and shift vars depending on the param it is called with. it should also handle the case where we need to add the starting point week instead of subtraction to handle the underflow

Testing

• Conduct forked tests to validate functionality and ensure correct values across a range of block times and scenarios.
• setL2TransitionBlock
• Calculate the correct newStartingPointWeek and set it
• Calculate the correct newShift and set it
• Write tests to cover: totalSupply and pastTotalSupply, balanceOf, Voting power and past voting power, lockedAmount and withdrawableAmount for multiple lock owners
• Update the _gap array to ensure storage slots are not affected

Acceptance Criteria:

• Locking contracts accurately handle both 5s and 1s block times.
• setL2TransitionBlock function successfully updates and registers L2 block number.
• roundTimestamp() correctly adjusts for week calculations post and pre transition.
• Tests confirm correctness across total supply, balances, voting power, locked and withdraw-able amounts for multiple lock owners at various points in time.