Detach dBFT extensible payloads verifier from dBFT engine

[AnnaShaleva]

Rationale

Extensible payloads verification does not depends on the dBFT engine in any way. The only common thing between them is the same algorithm for next block validators calculation. dBFT engine may be turned on/off (for non-mining/mining nodes), whereas extensible verification should always be enabled.

Implementation

Extensible payloads verification ((c *DBFT) IsExtensibleAllowed) should be moved to a separate tiny service by its own. This service should track Downloader events to properly react on chain sync, but it should not be a part of dBFT, the current design isn't good, ref. #298. Also, extensible verification service should have its own validators cache (#244).

[AnnaShaleva]

Pay attention to #298 (comment).