feat(subscribeorders.ts): SubscribeAddedOrders should have existing true by default

[ImmanuelSegol]

Closes #728

[kilrau]

Skimmed through the code, looks like

[kilrau]

api.md fine now. Rest needs to be reviewed & approved by either @sangaman or @moshababo

[sangaman]

#728 is not about adding a limit to the order subscription calls. Also, the way this is implemented it will return an arbitrary set of orders, which does not seem useful at all.

[ImmanuelSegol]

@sangaman How should I decide which orders should be returned?

[sangaman]

I just don't think we should be returning a subset of orders, and that's not the problem highlighted in the issue, so I think we should stick to returning all orders up front when existing is true.

[sangaman]

@ImmanuelSegol Will you be able to fix this typo soon? I think it makes sense to merge this before the next release and obviously the change is very minor, but if you're busy I can handle it.