Removed semaphore.wait() and introduced a timer pause boolean.

[Claeysson]

Potential fix for app hang caused by flushSync() in aptabaseClient.swift.

Removed semaphore.wait() and added a variable to pause the timer execution if flush isn't done executing.

closes #18

[goenning]

In this case there flushSync is no longer synonymous because it doesn’t wait for it to complete, maybe we could test just calling “flush” instead of “flushAsync” when the app goes to background or is closed?

[cristipufu]

I think we shouldn't block the app until the http call completes (unpredictable response time).

Maybe the sdk consumer can decide whether to call flush on applicationDidEnterBackground if needed (https://developers.google.com/analytics/devguides/collection/ios/v3/dispatch#override-applicationDidEnterBackground)

[cristipufu]

We will document a solution for this