MEDIUM: stats: implement a typed output format for stats

MEDIUM: stats: implement a typed output format for stats

The output for each field is :
  field:<origin><nature><scope>:type:value

where field reminds the type of the object being dumped as well as its
position (pid, iid, sid), field number and field name. This way a
monitoring utility may very well report all available information without
knowing new fields in advance.

This format is also supported in the HTTP version of the stats by adding
";typed" after the URI, instead of ";csv" for the CSV format.

The doc was not updated yet.