Improvements for verifier fork-choice & import-queue

[rphmeier]

Verifiers, being inherently parallelizable, should not be making decisions about fork-choice, since they don't have up-to-date information about the best block and any fork-choice that they make could be pre-empted by another import.

Two items:

• The verifier shouldn't have the option to return a fork-choice in BlockImportParams. fork-choice determination be done completely within the BlockImport structure, because it does sequential import of blocks.
• It would be nice for the verifier to return data that is passed through the pipeline to the block import down the line. Some investigation is needed to see how this can be done with minimal impact on BlockImport composability.