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.