Fix potential corruption/leak in cookiejar.

The flush() method of the cookie jar can be accessed from different
threads. This method (eventually) creates a NewRunnableTask and a
callback on FlushSemaphore object. Therefore FlushSemaphore should
inherit from a ThreadSafe ref counter rather than a simple base ref
counter.

Change-Id: Ie6914fba9b1554081c4b3b5992661af066e23653