Fix PaymentOptionsVC race condition where payment method loading race…

[yuki-stripe]

…s against setting defaultPaymentMethod.

We end up checking defaultPaymentMethod before initialization returns, leaving no room for the user to set the property.

This happens b/c

• We make a network request for payment methods in init
• That request can call the completion block synchronously if cached
• We don't clear the cache in PaymentOptionsVC (unlike PaymentContext)

I find all of this surprising but to avoid breaking other things, I'm leaving all of these things as-is and making the smallest fix.

Motivation

#1475

Testing

Tested on UI Examples app, which always synchronously returns. Repro'd pre-fix, cant repro post-fix.

[davidme-stripe]

A note from our offline chat about this: stpDispatchToMainThreadIfNecessary is kinda dangerous, as it isn't immediately obvious to the user whether it will execute immediately or on the next runloop. Changing stpDispatchToMainThreadIfNecessary to always call dispatch_async could cause side effects for existing users of the SDK, so we'll leave it alone for now, but we should probably reduce our usage of it.