Rewrite async I/O
Use the starting thread to wait for GUI input and instead use
the other threads to search. The consequence is that now think()
is alwasy started on a differnt thread than the caller that
returns immediately waiting for input. This reformat greatly
simplifies the code and is more in line with the common way
to implement this feature.
As a side effect now we don't need anymore Makefile tricks
with sleep() to allow profile builds.
No functional change.
Signed-off-by: Marco Costalba <mcostalba@gmail.com>