shared: thread safe initialization of nm_utils_get_monotonic_timestamp*()

nm_utils_get_monotonic_timestamp*() inherrently use static data. Let's
initialize it in a thread safe manner.

nm_utils_get_monotonic_timestamp*() are a fundamental utility function
that should work correctly in all cases. Such a low level function should
be thread safe.
5 jobs for th/monotonic-timestamp-cleanup in 8 minutes and 38 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #49653
checkpatch

00:01:03

passed #49654
f28_build_autotools

00:08:19

passed #49655
f28_build_meson

00:06:09

passed #49656
f28_rpm_autotools_dbg

00:08:37

passed #49657
f28_rpm_meson_dbg

00:07:21