State sync 1 - Blocks backfilling option

[abi87]

Why this should be merged

Counterpart of State sync - Blocks backfilling option

How this works

Missing stuff:

• [X] UTs
• [X] Add ETA to complete backfilling
• [X] What if state sync is re-run again before block backfilling completes? Current implementation would redownload a bunch of already downloaded blocks. Avoid that

How this was tested

Extra UTs + Fuji run (ongoing)