MINOR: stats: parse values from stats-file

MINOR: stats: parse values from stats-file

This patch implement parsing of counter values line from stats-file. It
reuses domain context previously set by the last header line. Each
value is separated by ',' character, relative to the list of column
names describe by the header line.

This is implemented via static function parse_stat_line(). It first
extract a GUID and retrieve the object instance. Then each numerical
value is parsed and object counters updated. For the moment, only U64
counters metrics is supported. parse_stat_line() is called on each line
until a new header line is found.