2-hop path-payments

[orbitlens]

Path payments just like on Stellar platform, but with a few essential distinctions:

1. Clients won't be allowed to specify path. Since Centaurus exchange supports only XLM markets, all payments will go through XLM: asset_a -> XLM -> asset_b
2. As we utilize the concept of lot size, it's possible that double conversion will produce "dust" leftovers of the intermediate asset (XLM) due to a rounding error. (Need to investigate this further).