Library: ensure thread-ID is initialised at thread start

While testing, I repeatedly had SEGFAULT in the new thread-wrapper,
but only when running under debugger. While the language spec guarantees
that exit from the thread handle initialisation synchronizes-with
the start of the new thread, there is no guarantee in the reverse
direction. Here this means that the new thread may not see the
newly initialised thread handle ID at start. Thus I've added
a yield-wait at the very beginning of the new thread function.

Under normal conditions, the startup of a thread takes at least
100 - 500µs and thus I've never seen the problematic behaviour
without debugger. However, adding a yield-wait loop at that point
seems harmless (it typically checks back every 400ns or so).

All real usages of the thread wrapper in the application use
some kind of additional coordination or even a sync barrier
to ensure the thread can pick up all further data before
going into active work.

WARNING: if someone would detach() the thread immediately after
creating it, then this added condition would cause the starting
thread function to hang forever. In our current setup for the
thread wrapper, this is not possible, since the thread handle
is embedded into protected code. The earliest point you could
do that would be in the handle_begin_thread(), which is called
from the thread itself *after* the new check. And moreover,
this would require to write a new variation of the Policy.

src/lib/thread.hpp

@@ -391,6 +391,8 @@ namespace lib {
         void
         invokeThreadFunction (ARGS&& ...args)
           {
+            while (not Policy::isLive()) // wait for thread-ID to become visible
+              std::this_thread::yield();//  (typically happens when debugging)
             Policy::handle_begin_thread();
             Policy::markThreadStart();
             Policy::perform_thread_function (forward<ARGS> (args)...);