Improve piece downloading

[nazar-pc]

There is some reafactoring here, but the primary goal is to accelerate PieceProvider.

Previously it had somewhat optimized implementation for downloading from connected peers and non-optimized implementation for closest peers downloads.

This new implementation is actually a bit simpler and unifies both mechanisms under single uniform logic. It does so by querying closest peers to convert them into connected and continue from there.

Together with piece cache tweaks from my testing this helps with both peak and sustained downloading speed. I'll be looking into avoiding making requests to connected peers that are not able to serve pieces and generally to non-farmers, but that will be a separate PR.

Code contributor checklist:

• I have read, understood and followed contributing guide

[shamil-gadelshin]

It makes sense overall but even after offline discussion it's hard to follow the logic. The comment for the changed function from piece_provider seems to obsolete. Please, try to summarize the current algorithm there.

[nazar-pc]

Cleaned up comment a bit. All crucial sections seem to have a comment describing what they do.