[ 2020-11-21; Reflector prioritization ]

Set priority to 3 during reflector discussions.

The timed lockable and mutex requirements currently use "rel_time denotes an object of an instantiation of duration, and abs_time denotes an object of an instantiation of time_point" to define the variables used to specify the timed lock functions. During LWG review of P2160R0, it was noted that this definition is deficient in two aspects:

• It doesn't allow for cv-qualification of rel_time and abs_time

• For time_points, it should require that the clock is a real Cpp17Clock. We impose that requirement via [thread.req.paramname], but there are no template parameters named Clock here.