This removes some needless local variables related to correction
field, but relying on a new "cField" entry in pp_instance, near
the t1,t2,t3,t4 status stamps.

The commit also adds a FIXME note: it is not clear to me how the
correction field can be managed in the back message (t3, t4).
Since the correctionField is updated by transparent clocks, I think
we need the one of the delay request (the one stamped in t3, t3),
*not* the delay response that I don't care about.
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>