Add _SingleActionPipeCondition class

This class will be used as a basic block for pipe(2)-based
conditions. Upon initialization it creates a pipe and can be
notified once (hence the “single action” in the name). A
callable helper class is used to wait for notifications.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>