Rename CWalletInterface to CValidationInterface

It's useful for much more than wallets.
Add CValidationInterface::BlockChecked notification
Bugfix: submitblock: Use a temporary CValidationState to determine accurately the outcome of ProcessBlock, now that it no longer does the full block validity check
Rename RPC_TRANSACTION_* errors to RPC_VERIFY_* and use RPC_VERIFY_ERROR for submitblock