Make Trade Target in swap optional

Trade Target is passed in the MEMO field and allows the price to be "protected". If the final price isn't met +/- 10% (TradeSlipLimit) then it is refunded.

However - this requires knowledge of the price in the first place, which requires querying for pool balances and computing the XYK formula to derive the price target.

We can only expect advanced user interfaces to go this length (such as our BEPSwap platform) where UX is paramount.

If a CLI or API-only based application attempted to swap, they will be swapping at "blind" prices - ie they don't care.

Thus Trade Target must be optional