Handle unprotected access to sending state in Emitter from concurrent threads (close #774)

[matus-tomlein]

Addresses issue #774 that reported a problem with concurrent access to the isSending property in Emitter which wasn't thread-safe.

To address the problem, this PR creates a wrapper class SendingLock around the isSending property that protects access to the value using a lock.

[hakonk]

Thanks for addressing the issue.

I like that the API surface has been reduced in size by making more methods private 👍.

As far as I can see, this makes access to isSending thread safe. That said, pausedEmit is not protected, because it can be accessed both outside and inside the lock of Emitter:334-342:

func resumeEmit() {
    pausedEmit = false
    flush()
}

/// Suspends sending events to collector.
func pauseEmit() {
    pausedEmit = true
}

and Emitter:374-383:

private func sendGuard() {
    if sendingLock.startSendingIfNotSending() {
        objc_sync_enter(self)
        if !pausedEmit {
            attemptEmit()
        }
        objc_sync_exit(self)
        sendingLock.stopSending()
    }
}

I have not managed to create a data race with the minimal example shown in #774, but both resumeEmit and pauseEmit are called from different places in the Snowplow library, so it seems it is possible.

Are there tests in place that assert that Emitter.flush still works as intended?

[hakonk]

LGTM!

[matus-tomlein]

Thank you for reviewing this @hakonk, very useful comments!