Implementing path finding without Horizon

[charlie-wasp]

This change is 

[nebolsin]

Added a few tini comments, otherwise

Reviewed 7 of 15 files at r1, 10 of 14 files at r2, 4 of 4 files at r3, 7 of 8 files at r4, 6 of 6 files at r5.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @charlie-wasp)

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src/service/dex/offers_listener.ts, line 31 at r5 (raw file):

      const offers = await getRepository(Offer).find({ where: pair });
      updateOffersGraph(pair.selling, pair.buying, offers);
    });

Shouldn't we use something like asyncForEach here? I think otherwise subscription callback can complete before those pair updates are still working.

// await asyncForEach(assetPairs, async (pair) => { ... })
export async function asyncForEach<T>(array: T[], mapper: (x: T) => Promise<void>): Promise<void[]> {
  return Promise.all(array.map(mapper));
}

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src/service/dex/offers_graph.ts, line 123 at r5 (raw file):

    //   return value;
    // }); // cloning

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