Fix pg_dump to read-lock all tables to be dumped as soon as it's read

Fix pg_dump to read-lock all tables to be dumped as soon as it's read
their names from pg_class.  This considerably reduces the window wherein
someone could DROP or ALTER a table that pg_dump is intending to dump.
Not a perfect solution, but definitely an improvement.  Per complaints
from Marc Fournier; patch by Brent Verner with some kibitzing by Tom Lane.