OrderBook: allow different quote assets

[wer1st]

Now we are following the logic when only the dex base asset could be the quote asset of all order-books in this dex. In practice it is XOR (and maybe XSTUSD if we allow dex_id = 1). Probably we can allow not only dex base assets as quote asset. For example ETH / DAI, AAVE / XSTEUR etc. Such pairs could increase the user interest to use Polkaswap.

@Tieumsan please review