We do not want a value as large as the previous one, which limits the time
range that can be represented in 62 bits unnecessarily. The new value is
9 times smaller than the previous value, and loses only 384000 as a significant
factor.
This commit also switches to using an (inline) accessor for superclock_ticks_per_second,
making it possible in debug/testing phases to spot early/illegal uses of the value.