fix(libs/header/sync): Make ranges.Add thread-safe

[tzdybal]

Overview

ranges.Add method is not thread safe. if you execute it in parallel you can end up with ranges out-of-order.
In syncer.newNetHead nothing prohibits parallel execution of pending.Add - syncing point is in wantSync using buffered channel.
newNetHead is called ass libp2p gossip validator, which is asynchronous - can be executed in parallel.

This change ensures that Add method is thread-safe.
Added test fails in first commit (to proof the point), and second commit fixes the issue.

Probably fixes #955. My gut feeling is that is also may fix other, more-nuanced, non-panicing sync issues.

Checklist

• New and updated code has appropriate documentation
• New and updated code has new and/or updated testing
• Required CI checks are passing
• Visual proof for any user facing features like CLI or documentation updates
• Linked issues closed with keywords

[Wondertan]

Can you try to run the reconstruction swamp teat locally with -count 100 to see if it's really fixes it?

Good catch anyway!

[renaynay]

Looks fine to me - am running it against arabica where a lot of these issues were observed and looks good so far.

Thanks for the fix @tzdybal

[renaynay]

https://github.com/celestiaorg/celestia-node/actions/runs/4023389491/jobs/6914202398#step:4:320