pgsql: pg_dump: Further reduce reliance on global variables.

pg_dump: Further reduce reliance on global variables.

This is another round of refactoring to make things simpler for parallel
pg_dump. pg_dump.c now issues SQL queries through the relevant Archive
object, rather than relying on the global variable g_conn. This commit
isn't quite enough to get rid of g_conn entirely, but it makes a big
dent in its utilization and, along the way, manages to be slightly less
code than before.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/1631598ea204a3b05104f25d008b510ff5a5c94a

Modified Files
--------------
src/bin/pg_dump/pg_backup_archiver.c | 10 +
src/bin/pg_dump/pg_backup_archiver.h | 1 +
src/bin/pg_dump/pg_backup_db.c | 24 ++
src/bin/pg_dump/pg_backup_db.h | 4 +
src/bin/pg_dump/pg_dump.c | 449 ++++++++++++++--------------------
5 files changed, 218 insertions(+), 270 deletions(-)