Test payment syncs

[nieznanysprawiciel]

Fixes:

• Send Invoice acceptance even if it is already settled
• Don't clear sync_notifs table after processing each owner identity (could result in not synchronized entities)
• Fix ya-client-model dependency
• Use signed_bytes and validate content against Payment struct sent by Requestor

Requires:

• Add get_signed_payment endpoint ya-client#193
• Make ya-client-model dependency more flexible golem-certificate#39