Add Gateway method for reducing the size of a position

[flanagansteve]

Sometimes, you can't close a position, and burnOptions fails, because someone has bought a small amount of your position. In those cases, we should allow the bot to reduce the size of the position (if the gas costs justify it) - for example, if you were selling 10 ETH worth of calls and someone bought 0.01 ETH, having 10ETH that is un-burnable is a pretty big opportunity cost - but reducing the size of the position will only cost some gas while freeing that capital.

We should think about how this trades off against:

• gas cost
• time - maybe if you waited a bit the conditions would change and you wouldn't need to reduce size at all
• force exercising - maybe paying the force exercise fee is worth it in exchange for freeing all your capital

But, when it makes sense, we should use this on-tick in strategies when a simple burnOptions call fails (for example, when you're trying to close an out-of-range position to re-open in-range ones)

Can look to the logic in the UI for design inspiration on how to reduce size