Rewrite data model and interface for token-vesting program

[s-medvedev]

The data model of the original contract involves storing the balance to which the funds will be withdrawn. Using this data, the contract allows any user to withdraw funds, as well as the owner of the balance to set a new withdrawal address.
However, the account owner can be changed using the spl-token program. This change can be made outside of the vesting contract and violates the integrity of the user's vote weight data in the governance contract.

To ensure data integrity, it was decided to change the contract data model, as well as the method interface.

Changes to be made:

1. store the owner in the vesting record, not the withdrawal address
2. change the list of accounts in the instructions in accordance with the changes in the model
3. replace the ChangeDestination instruction with ChangeOwner

┆Issue is synchronized with this Jira Task by Unito