MINOR: stats: introduce a more expressive stat definition method

MINOR: stats: introduce a more expressive stat definition method

Previously, statistics were simply defined as a list of name_desc, as
for example "stat_cols_px" for proxy stats. No notion of type was fixed
for each stat definition. This correspondance was done individually
inside stats_fill_*_line() functions. This renders the process to
define new statistics tedious.

Implement a more expressive stat definition method via a new API. A new
type "struct stat_col" for stat column to replace name_desc usage is
defined. It contains a field to store the stat nature and format. A
<cap> field is also defined to be able to define a proxy stat only for
certain type of objects.

This new type is also further extended to include counter offsets. This
allows to define a method to automatically generate a stat value field
from a "struct stat_col". This will be the subject of a future commit.

New type "struct stat_col" is fully compatible full name_desc. This
allows to gradually convert stats definition. The focus will be first
for proxies counters to implement statistics preservation on reload.