Add missing deflateEnd() for server-side gzip base backups

Add missing deflateEnd() for server-side gzip base backups

The gzip basebackup sink called deflateInit2() in begin_archive() but
never called deflateEnd(), leaking zlib's internal compression state
(~256KB per archive) until the memory context of the base backup is
destroyed.

The code tree has already a matching deflateEnd() call for each
deflateInit[2]() call (pgrypto, etc.), except for the file touched in
this commit, so this brings more consistency for all the compression
methods.  The server-side LZ4 and zstd implementations require a
dedicated cleanup callback as they allocate their state outside the
context of a palloc().

As currently used, deflateInit2() is called once per tablespace in a
single backup.  Memory would slightly bloat only when dealing with many
tablespaces at once, not across multiple base backups so this is not
worth a backpatch.  This change could matter for future uses of this
code.

zlib allows the definition of memory allocation and free callbacks in
the z_stream object given to a deflateInit[2]().  The base backup
backend code relies on palloc() for the allocations and deflateEnd()
internally only cleans up memory (no fd allocation for example).

Author: Jianghua Yang <yjhjstz@gmail.com>
Discussion: https://postgr.es/m/CAAZLFmQNJ0QNArpWEOZXwv=vbumcWKEHz-b1me5gBqRqG67EwQ@mail.gmail.com