More vacuum stats

I noticed that we were already tracking the information about when an
autovacuum worker was last started in a database, but this information
was not exposed. The attached patch puts this column in
pg_stat_database.

Was there any particular reason why this wasn't exposed before that
I've missed, making this a bad addition? :-)

Oh, and this time, I *have* included updates to the regression tests.

-- 
 Magnus Hagander
 Me: http://www.hagander.net/
 Work: http://www.redpill-linpro.com/