The function pointer ut_timer() was only used by the
InnoDB defragmenting thread. Let InnoDB use a single monotonic
high-precision timer, my_interval_timer() [in nanoseconds],
occasionally wrapped by microsecond_interval_timer().

srv_defragment_interval: Change from "timer" units to nanoseconds.

This concludes the InnoDB time function cleanup that was
motivated by MDEV-14154. Only ut_time_ms() will remain for now,
wrapping my_interval_timer().