GateIO GetOpenInterest returning notional value instead

[gbjk]

[...] This looks like notional value not open interest. Open interest can be derived from GetFutureStats this call

Need to switch to using GetFutureStats.

Probably fix as with a dependency stack on top of #1786, even though this is a bug, because otherwise would need completely divergent solutions.

Originally posted by @shazbert in #1786 (comment)

[gbjk]

This isn't a slam dunk; There's no futures stats available for Delivery Futures.
Also going to have to iterate all contracts.