In previous implementation `verifySignature` method of container
processor worked incorrectly for operations without a key and with
session: processor tried to verify signature with one of the bound owner
keys instead of session one.
Use `VerifySessionDataSignature` method to check the signature if
session is used. Refactor `verifySignature` a bit with session check
highlighting for readability.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
fa009db140