Fix args memory leak in rows.Close()

[hrubymar10]

Hello,
I have found the memory leak in rows.Close() method. Please check it out.

The code for reproduction is available here: https://github.com/hrubymar10/pgx-memory-leak-test/tree/master

heap pprof before fix:

heap pprof after fix:

pprof files:
heap.pb.zip
heap_fixed.pb.zip

[jackc]

If setting rows.args to nil allows it to be GCed, then that implies that rows cannot be GCed for some reason. That would seem to be the ultimate issue.

It looks like the preallocated buffer of poolRows in pgxpool.connResource. That saves a alloc per query, but it pins the underlying row until the preallocated buffer is GCed.