VaultClawback: Burn shares of an empty vault (XRPLF#6120)

- Adds a mechanism for the vault owner to burn user shares when the vault is stuck. If the Vault has 0 AssetsAvailable and Total, the owner may submit a VaultClawback to reclaim the worthless fees, and thus allow the Vault to be deleted. The Amount must be left off (unless the owner is the asset issuer), specified as 0 Shares, or specified as the number of Shares held.

Author: Tapanito