Proposed patch: synchronized_scanning GUC variable

Per today's -hackers discussion, add a GUC variable to allow clients to
disable the new synchronized-scanning behavior, and make pg_dump disable
sync scans so that it will reliably preserve row ordering.  This is a
pretty trivial patch, but seeing how late we are in the 8.3 release
cycle, I thought I'd better post it for comment anyway.

			regards, tom lane