Wait for main thread to finish before to exit

Wait for main thread to finish before to exit

Currently after a 'quit' command UI thread raises stop
signal, exits from uci_loop() and calls Threads.exit()
while the search threads are still active.

In Threads.exit() main thread is asked to terminate, but
if it is parked in idle_loop() it will exit and free its
resources (in particular the shared Movepicker object) while
sibling slaves are still active and this leads to a crash.

The fix is to let the UI thread always wait for main thread
to finish the search before to return from uci_loop().

Found by Valgrind when running with 8 threads.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>