In psql \copy from, send data to server in larger chunks.

In psql \copy from, send data to server in larger chunks.

Previously, we would send each line as a separate CopyData message.
That's pretty wasteful if the table is narrow, as each CopyData message
has 5 bytes of overhead. For efficiency, buffer up and pack 8 kB of
input data into each CopyData message.

The server also sends each line as a separate CopyData message in COPY TO
STDOUT, and that's similarly wasteful. But that's documented in the FE/BE
protocol description, so changing that would be a wire protocol break.

Reviewed-by: Aleksander Alekseev
Discussion: https://www.postgresql.org/message-id/40b2cec0-d0fb-3191-2ae1-9a3fe16a7e48%40iki.fi