Fold vp_validator into vp_user & rework init validator

[cwgoes]

• Fold vp_validator into vp_user and Delete vp_validator: combine user vp and validator vp #2202
• Update the init validator transaction so that it no longer creates a new account, but rather using an existing account (the source of the transaction): replace tx_init_validator with tx_become_validator that doesn't create an account #2208
  • Initially require that there are no existing delegations from this account to avoid potential issues as the system was built with assumption that validators cannot delegate
• Check whether we have some assumptions about validators not delegating to other validators
  • If there are no issues, we can relax init-validator to allow using accounts that have existing delegations
  • Update is_delegator, is_validator, etc.