Proxy contract used by notary contract as a main tx
payment provider. `Verify` function verifies if
main tx contains multisig of 5\7 inner ring
nodes. If so, then contract pays for transaction.
This is easier than support multisig wallet with
GAS assets, because inner ring can change, therefore
multisig wallet can change periodically.
Signed-off-by: Alex Vanin <alexey@nspcc.ru>
