[WIP] Fast Sync

[willmeister]

Implements Fast Sync as described by #637

All that is left for this PR is:

• Add tests
• Add fast sync eligibility / configuration detection and set BlockQueue.fast_sync_in_progress accordingly
• Determine the chain's block height, flip fast_sync_in_progress to false and load necessary tries when we reach block height - 1024
• Validation for Receipts received against block header hash

[InoMurko]

👍