firebase-ios-sdk

iOS: fix data race in FSTDispatchQueue. (#1070)

operationInProgress is accessed from both the main thread and from
libdispatch on some other thread. Make it atomic to avoid a data race.
Also reorder assertion checks to only access operationInProgress
after making sure the function is running on the queue.

Tested: ran unit tests using old and new versions under Thread
Sanitizer, verified that TSan reports a data race for the old version,
but finds no issues with the new version.