Print more details about the warp syncing while it is in progress

[tomaka]

As evidenced by https://github.com/paritytech/substrate-connect/issues/927, it happens that warp sync just takes ages. Right now we print a periodic warning saying "don't worry, we're still alive". This message should include some information or statistics about the progress of the warp syncing.

[tomaka]

Accidentally opened in the wrong repo.

[melekes]

Are there any specific data points that we want to output?

[tomaka]

The warp syncing consists in multiple stages: the first one (normally the longest) consists in downloading and verifying the fragments to do the actual syncing, and the rest consists in some extra network requests to download the state at the head of the chain (without which we couldn't progress from there).

I think that we should print the stage we're at, and for the download stage print the block number of the latest fragment that we've downloaded.

[tomaka]

Note that the warp sync code at the moment downloads all the fragments (this usually takes multiple requests) then verifies them all at once.

It would be better if fragments were verified progressively (verify the current batch of fragments while the next batch is being downloaded), both to speed things up but also to show with certainty the block number that we've synced to.