MINOR: stats: parse header lines from stats-file

MINOR: stats: parse header lines from stats-file

This patch implements parsing of headers line from stats-file.

A header line is defined as starting with '#' character. It is directly
followed by a domain name. For the moment, either 'fe' or 'be' is
allowed. The following lines will contain counters values relatives to
the domain context until the next header line.

This is implemented via static function parse_header_line(). It first
sets the domain context used during apply_stats_file(). A stats column
array is generated to contains the order on which column are stored.
This will be reused to parse following lines values.

If an invalid line is found and no header was parsed, considered the
stats-file as ill formatted and stop parsing. This allows to immediately
interrupt parsing if a garbage file was used without emitting a ton of
warnings to the user.