System.Web.SessionState: simplify sessions retrival polling

When using "fat" session (~2MB) and multiple (~20) simultaneous calls
the retrieval sometimes fails and lock the session.
The original code blocks on the calling thread (AutoResetEvent.WaitOne())
and uses another thread with the timer to do the polling.
If the polling succeeds, the TimerCallback AutoResetEvent.Set(),
with no guarantes that it will wake up the thread.
(see Remarks in http://msdn.microsoft.com/en-us/system.threading.eventwaithandle.set%28v=vs.110%29)

In this patch we simply replace all this code with an equivalent
loop + sleep, which give us the same behaviour
(we still block the calling thread with the sleep, but we didn't use another one)

Signed-off-by: Etienne CHAMPETIER <etienne.champetier@fiducial.net>