pg_autovacuum fixes

This weekend I am trying to fix up all known the pg_autovacuum issues
that should be resolved for 7.4.3. I am aware of only two issues: temp
table issues, and unchecked send_query() calls, if I am forgetting
something, please let me know.

1) temp table issue:
I was not able to reproduce the crash associated with temp tables. I
spent a while creating tables doing updates and dropping them trying
without success to get pg_autovacuum to crash. Since I couldn't trigger
the problem, I will need someone else to test to see if I have fixed the
problem. Anyway, I have modified the query to exclude temp tables from
the list of tables to work with. So we should no longer be dealing with
temp tables at all which should side step any temp table related problem
we might have been having.

2) Unchecked send_query() function calls:
As best as I can tell, this is mostly a non-issue, but I went ahead
added a check to any section that did anything with the result of
send_query, so if this was an issue, it should be fixed now. BTW, this
might have been the cause of the temp table related crash, but that is
just a guess.

Matthew O'Connor