FxDAO returns, with a shape built for vaults

FxDAO rejoins Blend, Aquarius and Soroswap in the Positions API, reshaped around what a vault actually is rather than the pool-share form the other three share.

• A vault comes back with its collateral and its outstanding debt kept apart, in the denomination it was opened in — US dollars, euros or pounds.
• Pin a request to a ledger and the API answers with the vaults as they stood at that point, which is what a reconciliation or a report needs. No vault on the network is open today, so the history is where the value sits.
• A vault is open when it still holds collateral. A closed one is never returned as an open position, and a wallet holding none comes back empty rather than as an error.
• USD values are attached where a price resolves and are null with the reason named where none does, never a fabricated zero.