Split copy.c into four files.

Split copy.c into four files.

Copy.c has grown really large. Split it into more manageable parts:

- copy.c now contains only a few functions that are common to COPY FROM
  and COPY TO.

- copyto.c contains code for COPY TO.

- copyfrom.c contains code for initializing COPY FROM, and inserting the
  tuples to the correct table.

- copyfromparse.c contains code for reading from the client/file/program,
  and parsing the input text/CSV/binary format into tuples.

All of these parts are fairly complicated, and fairly independent of each
other. There is a patch being discussed to implement parallel COPY FROM,
which will add a lot of new code to the COPY FROM path, and another patch
which would allow INSERTs to use the same multi-insert machinery as COPY
FROM, both of which will require refactoring that code. With those two
patches, there's going to be a lot of code churn in copy.c anyway, so now
seems like a good time to do this refactoring.

The CopyStateData struct is also split. All the formatting options, like
FORMAT, QUOTE, ESCAPE, are put in a new CopyFormatOption struct, which
is used by both COPY FROM and TO. Other state data are kept in separate
CopyFromStateData and CopyToStateData structs.

Reviewed-by: Soumyadeep Chakraborty, Erik Rijkers, Vignesh C, Andres Freund
Discussion: https://www.postgresql.org/message-id/8e15b560-f387-7acc-ac90-763986617bfb%40iki.fi