feat: Global delayed insurance withdrawal

[OsmanBran]

Motivation

Delayed insurance withdrawal should be enforced on all withdrawals to prevent sudden draining of the insurance pool.

When a user wants to withdraw from the pool they should first call intendToWithdraw. They will then have to wait for the cooldown period to elapse (5 days). Then, they will have a window of time to withdraw (2 days). Once this passes they will have to renotify their intent to withdraw before withdrawing.

          ┌───────────────────────────────────┬─────────────────────────┐
          │      `COOLDOWN_PERIOD` 5 days     │ `WITHDRAW_WINDOW` 2 days│
          └───────────────────────────────────┴─────────────────────────┘
           ↑                                               ↑ 
           User calls `intendToWithdraw`        User can call `withdraw`
           User can call `cancelWithdraw` to reset cooldown

Note that there is no on-chain bookkeeping for delayed withdrawals, pool collateral is only updated once the user has called withdraw.

Changes

• Add intendToWithdraw, cancelWithdraw and update withdraw functions as above
• Added tests