If the QBindingStatus we receive from the QObject is from the thread
which is currently running, we do not need to refetch the thread local;
the reason we refetched the thread_local was after all only to guard
against the case where a QObject's property gets read in a different
thread.
To determine whether we are in the same thread, we can store the thread
id in the thread_local structure. Currently, it only gets initialized
for the main thread in QCoreApplication (as bindings are mostly used
there).
At a later point, we could either expose initBindingStatusThreadId, or
call the function when a QThread gets started.

Pick-to: 6.2
Change-Id: Id8eb803973bb083abfab83a62bfccc9e88a4e542
Reviewed-by: Lars Knoll <lars.knoll@qt.io>