Use all pairs for conversions

https://github.com/ARK-Builders/ARK-Rate/pull/13/files/3ab38d7d0f286ad24a388faa69d251fcff49dfb7#diff-3356bc53a97bf9b7871002de7206cd61868a871759da3c49c1d29fa3dfba4c52R40

When we convert crypto assets, we use only those pairs, which have USD as one side of the pair. In other words, we always convert one crypto token to another via USDT. We could achieve more realistic conversion if we store all pairs in memory and first attempt to find direct match, before converting via USDT as a fallback.

Bonus points for advanced routing, e.g. `ATOM` -> `DOT` -> `BTC`.

Start with the conversion code shown in pull request #13's linked diff, around the referenced line 40, and trace how pairs are stored and selected. Done means direct crypto pairs are tried before the USDT fallback; advanced multi-hop routing is an optional extension mentioned by the issue.

Written by the indexing model from the issue text.