Verify correct payloads returned via engineAPI

[NiloCK]

Monomer's Payload type has a helper to construct ExecutionPayloads for consumption over the engineAPI.

When writing the executionPayload for a block, the helper uses the payload's transaction list.

This is potentially misleading / inaccurate, because the block itself is built with transactions sourced from this payload object and also the mempool.

Should the returned ExecutionPayload be including this mempool tx data?