Sending messages to a subscriber from different threads can result in delivery race conditions

[jspahrsummers]

As pointed out by @Coneko in #136, sending next, error, or completed to a RACSubscriber or RACSubject from multiple threads might result in them being processed simultaneously, and most <RACSignal> and <RACStream> methods aren't built to handle it.

Subjects and subscribers should serialize their messages onto a private queue so that stream methods don't have to worry about synchronization.

[dannygreg]

To me it was always inferred that the messages would be serialised simply from the API. Definitely 👍