Rename flag 'stop' in 'stopRequest'

Instead of other flags this is not a state flag, i.e. does
not defines a state for the thread, but a request because
after we raise 'stopRequest' flag the corresponding thread is
not stopped, but continues to run for a while until it returns
from sp_search() in idle_loop.

It is important the name reflects this.

No functional change.

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

src/thread.h

@@ -70,7 +70,7 @@ struct Thread {
   volatile int activeSplitPoints;
   uint64_t nodes;
   uint64_t betaCutOffs[2];
-  volatile bool stop;
+  volatile bool stopRequest;
   volatile bool running;
   volatile bool idle;
   volatile bool sleeping;