time: Have skewing-now call non-skewing now

time: Have skewing-now call non-skewing now

For the real-time clocks, Ceph's testing infrastructure likes to be able to
inject a skew. To avoid pulling CephContext into ceph_time.h these are moved to
ceph_time.cc. The original way this was done called clock_gettime in both
places.

This is an unnecessary duplication and apparently error-prone. So only call
clock_gettime from one place.

Signed-off-by: Adam C. Emerson <aemerson@redhat.com>