bug (perp): can't open position on master when there's more than 1 vpool

[matthiasmatt]

On master currently opening a position is broken if there's more than one vpool. The tx fails with the message error updating reserve: price is over fluctuation limit.

Reason is that the checkFluctuationLimitRatio function look for a snapshot to compare with, but the GetLatestReserveSnapshot function returns a random vpool snapshot:

// GetLatestReserveSnapshot returns the last snapshot that was saved
func (k Keeper) GetLatestReserveSnapshot(ctx sdk.Context, pair common.AssetPair) (
	snapshot types.ReserveSnapshot, err error,
) {
	store := prefix.NewStore(ctx.KVStore(k.storeKey), types.SnapshotsKeyPrefix)
	iter := store.ReverseIterator(nil, nil)

	defer iter.Close()

	for ; iter.Valid(); iter.Next() {
		k.codec.MustUnmarshal(iter.Value(), &snapshot)
		return snapshot, nil
	}

	return types.ReserveSnapshot{}, types.ErrNoLastSnapshotSaved
}

This results on using the eth:usd vpool to check the fluctuation limit of the btc:usd vpool:

price 20000.000000004000000000
lastPrice 1500.001500001500001500
upperLimit 1650.001650001650001650
lowerLimit 1350.001350001350001350
price.GT(upperLimit) true
price.LT(lowerLimit) false