MEDIUM: stats: convert counters to new column definition

MEDIUM: stats: convert counters to new column definition

Convert most of proxy counters statistics to new "struct stat_col"
definition. Remove their corresponding switch..case entries in
stats_fill_*_line() functions. Their value are automatically calculate
via me_generate_field() invocation.

Along with this, also complete stcol_hide() when some stats should be
hidden.

Only a few counters where not converted. This is because they rely on
values stored outside of fe/be_counters structure, which
me_generate_field() cannot use for now.