Add per-epoch withdraw caps

[aoikurokawa]

Motivation

• Closes #3

Solution

• Add epoch_withdraw_cap, current_epoch, epoch_withdrawn_amount in Vault struct.
• Does not allow withdrawing over epoch_withdraw_cap

[aoikurokawa]

@buffalu Can you review this PR? Thank you so much!

[coachchucksol]

Hey @aoikurokawa! Thanks for the contribution!

We've had some discussions around this and need the solution to #3 to be as follows:

The vault struct needs the following: epoch_withdraw_cap_bps, last_slot_updated, epoch_withdraw_amount, epoch_snapshot_amount along with the function is_update_needed

• epoch_withdraw_cap_bps - the percentage 25% - 100% (2500 - 10000) that is the max that can be withdrawn from the vault based on a snapshot of assets in the vault at the beginning of an epoch
• last_slot_updated - The slot when process_update_vault is called at the beginning of an epoch
• epoch_withdraw_amount - The tally of assets withdrawn on that epoch, this cannot be above epoch_snapshot_amount x epoch_withdraw_cap_bps
• epoch_snapshot_amount - The amount of assets in the vault at the time of calling process_update_vault
• is_update_needed - Check that Clock.slots / config.epoch_length ( Seeis_update_needed in VaultDelegationList )

Flow:

At the beginning of the epoch ( When vault.is_update_needed is true ) we run process_update_vault this will:

• Set last_slot_updated to Clock.slot
• Clear epoch_withdraw_amount to 0
• Set epoch_snapshot_amount to whatever is available to withdraw from the vault

When a burn or enqueue withdrawal ticket happens:

• Check that vault.is_update_needed is false
• Check that epoch_withdraw_amount + amount_to_withdraw <= epoch_snapshot_amount * epoch_withdraw_cap_bps
• Increment epoch_withdraw_amount by amount_to_withdraw

Hope this all makes sense!

[aoikurokawa]

@buffalu @coachchucksol Thank you for explanation! Ready for review it! Thank you!

[coachchucksol]

Hi! I'll take a look at it today!

[aoikurokawa]

@coachchucksol Thank you for reviewing!!

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 82.44275% with 23 lines in your changes missing coverage. Please review.

Project coverage is 83.51%. Comparing base (4af6d94) to head (fde61e1).

Files with missing lines	Patch %	Lines
vault_core/src/vault.rs	79.27%	23 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #29 +/- ## ========================================== + Coverage 82.29% 83.51% +1.22% ========================================== Files 86 86 Lines 8651 8780 +129 ========================================== + Hits 7119 7333 +214 + Misses 1532 1447 -85 ```

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