Use block's number for heightToDownload when downloading headers

[mininny]

Currently, blockDownloader uses 0 as a hard-coded value for heightToDownload. This isn't necessarily a problem because we are using hashToDownload to download the headers.

I modified the code to use block.Number() for heightToDownload because:

1. in some random cases where hashToDownload is empty
2. we can see blockHeight in logs rather than just hashes
3. otherwise, we can also choose to just use 0 as default value when the hash is not available and remove unnecessary code.

I'd be open to go to #3 and rewrite this PR if you think it's more appropriate :)

[yperbasis]

Closing since it's very old