pgsql: Refactor the pg_dump zlib code from pg_backup_custom.c to a sepa

Refactor the pg_dump zlib code from pg_backup_custom.c to a separate file,
to make it easier to reuse that code. There is no user-visible changes.

This is in preparation for the patch to add a new archive format, a directory,
to perform a custom-like dump but with each table being dumped to a separate
file (that in turn is a prerequisite for parallel pg_dump). This also makes it
easier to add new compression methods in the future, and makes the
pg_backup_custom.c code easier to read, when the compression-related code is
factored out.

Joachim Wieland, with heavy editorialization by me.

Branch
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master

Details
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http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=bf9aa490db24b2334b3595ee33653bf2fe39208c

Modified Files
--------------
src/bin/pg_dump/Makefile | 2 +-
src/bin/pg_dump/compress_io.c | 403 ++++++++++++++++++++++++++++++++++
src/bin/pg_dump/compress_io.h | 68 ++++++
src/bin/pg_dump/pg_backup_archiver.h | 7 +-
src/bin/pg_dump/pg_backup_custom.c | 365 ++++++------------------------
5 files changed, 552 insertions(+), 293 deletions(-)