Fix permanent memory leak in autovacuum launcher

get_database_list was uselessly allocating its output data, along some
created along the way, in a permanent memory context.  This didn't
matter when autovacuum was a single, short-lived process, but now that
the launcher is permanent, it shows up as a permanent leak.

To fix, make get_database list allocate its output data in the caller's
context, which is in charge of freeing it when appropriate; and the
memory leaked by heap_beginscan et al is allocated in a throwaway
transaction context.