Bug 7785: remove MySQL-specific syntax from C4::Members::columns()

The initial thought was to remove this function. However,
tools/import_borrowers.pl uses it. So rather than remove
it to solve the problem, it was reworked to a more generic
solution which runs faster.

By accessing $sth->{NAME} directly, the driver becomes
responsible for filling it correctly. This happens when a SELECT
is done on the borrowers table. It does not even have to have
data in the result set!

The columns method could be more generic and used elsewhere too.
Comparison between the old method and the STH method showed a
significant time difference. The old method took 35 seconds
for 40k iterations versus 19 seconds for the same amount of
iterations with the STH method regardless of the size of the
borrowers table.

Signed-off-by: Srdjan <srdjan@catalyst.net.nz>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Bug 7785: (follow-up) standardize POD

This makes the POD for the columns() function consistent
with the rest of C4/Members.pm.  It also removes a note
that can be relegated to the bug report and the Git
history.

Also, since C4::Members::columns() is not actually a
class method, this patch changes the invocation to
not call it that way.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>