Add greedy withdrawal allocation

[buffalu]

• Withdrawals need to be resistant to the delegation manager shutting down so funds don't get stuck.
• On epoch updates, if the delegation manager hasn't withdrawn funds for the VRTs pending redemption, this code will do it automatically.

Other things:

• Removes the concept of withdraw funds in the delegation state. Not needed anymore.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 94.74940% with 22 lines in your changes missing coverage. Please review.

Project coverage is 76.23%. Comparing base (daa2c15) to head (4371846). Report is 1 commits behind head on master.

Files	Patch %	Lines
vault_core/src/vault_update_state_tracker.rs	87.71%	7 Missing :warning:
...lt_program/src/crank_vault_update_state_tracker.rs	85.00%	6 Missing :warning:
vault_core/src/vault.rs	98.12%	5 Missing :warning:
vault_core/src/vault_operator_delegation.rs	82.35%	3 Missing :warning:
vault_sdk/src/instruction.rs	83.33%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #56 +/- ## ========================================== - Coverage 76.81% 76.23% -0.58% ========================================== Files 83 83 Lines 7676 7483 -193 ========================================== - Hits 5896 5705 -191 + Misses 1780 1778 -2 ```

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