MINOR: stats: support age in stats-file

MINOR: stats: support age in stats-file

Extend generic stat column support to be able to fully support age stats
type. Several changes were required.

On output, me_generate_field() has been updated to report the difference
between the current tick with the stored value for FN_AGE type. Also, if
an age stats is hidden in show stats, -1 is returned instead of an empty
metric, which is the value to mark an age as unset.

On counters preload, load_ctr() was updated to handled FN_AGE. A similar
substraction is performed to the current tick value.