Ensure the COPY fetcher implementation reads data until EOF with
`PQgetCopyData()`. Also ensure the malloc'ed copy data is freed with
`PQfreemem()` if an error is thrown in the processing loop.
Previously, the COPY fetcher didn't read until EOF, and instead
assumed EOF when the COPY file trailer is received. Since EOF wasn't
reached, it required terminating the COPY with an extra call to the
(deprecated) `PQendcopy()` function.
Still, there are cases when a COPY needs to be prematurely terminated,
for example, when querying with a LIMIT clause. Therefore, distinguish
between "normal" end (when receiving EOF) and forceful end (cancel the
ongoing query).