assetPairs endpoint - pools

[AusIV]

One feature pools should eventually have is to let the pool specify the asset pairs in the assetPairs endpoint. Right now, pairs is based on a fairly convoluted query to determine all of the pairs that have been listed. Because of how this is structured, it would be difficult to change it to filter based on what pairs have been listed in a given pool.

A simpler implementation of asset pairs would be to have a table enumerating pairs. We could populate that as we save new orders, and could attach a pool identifier to the items in the table to allow filtering by pool.