share(replay: 2+, scope: .whileConnected) misses events in multithreaded environment

[isaac-weisberg]

I have been writing unit tests for #2654 and found a bug in ConnectableObservableAdapter:

• Multicast operator produces a ConnectableObservableAdapter
• when you subscribe to it via Observable interface, it evaluates lazySubject computed property to instantiate a Subject without synchronization
• this can cause the makeSubject to be called more than once if you perform several calls to subscribe concurrently
• and if it's going to return different instances of Subject, this means that some of the concurrent subscriptions will actually be looking at the wrong instance of Subject, which will be overwritten during the data race.

This setup will naturally occur if:

• you are doing flatMap into a connectable observable concurrently
• if you use refCount operator, subscribing concurrently
• share(replay: 2+, scope: .whileConnected) because it's implemented via multicast + refCount

You can see the code that reproduces this issue here: https://github.com/isaac-weisberg/RxSwiftDeadlock/tree/connect-unsafe

I am going to provide a fix for this issue inside #2654 since it's required to make all the tests run successfully.