Add block number to block root index

[MegaRedHand]

For blocks-by-range requests we need to find the blocks for the given slots. We should add an index to our DB so the search is efficient, instead of the current sequential loop over all blocks.

[dicethedev]

For blocks-by-range requests we need to find the blocks for the given slots. We should add an index to our DB so the search is efficient, instead of the current sequential loop over all blocks.

@MegaRedHand you can assign this to me since I work on #348