Don't create as many garbage objects during reads

The old method of reading a row by creating a new reader for every row is
quite inefficient because it leaves a lot of objects laying around for the
garbage collector.  This change creates a CSVParser once and then re-uses it
for every row.

There's a chance this change causes a regression, because CSVSerde is no
longer serializable (CSVParser isn't serializable) but this doesn't seem to be
an issue for me in my testing.