Validate any spawned commit transaction prior adding them to the wallet

[frolosofsky]

As has been described in the https://github.com/dtr-org/unit-e/issues/996#issuecomment-483694251, node could keep invalid spawned finalizer commit transactions in the wallet because there's a lack of validation of them prior adding to the wallet.

This could be fixed in a similar way as #1040 does it for deposit transactions. @kostyantyn also came with a proposal to use ContextualCheck: https://github.com/dtr-org/unit-e/pull/1040#discussion_r280456698. 3rd option is to apply the full transaction validation path.

User-driven commits (definitely must be fixed):

• [ ] deposit #1040
• [ ] logout
• [ ] withdraw
• [ ] admin

System-driven commits (needs more investigation):

• [ ] vote
• [ ] slash