Rename flag 'stop' in 'stopRequest'

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>