slightly changes definition so that autovacuum works parallel safe too
Fixes #540 (closed)
Basic issue is that the autvacuum worker executes in a different security context. This marks the function such that it executes as the creator of the function, which will be the db owner, such that the "users" table is visible and accessible during automatic vacuum/analyze.
Additionally this marks the function as parallel safe, thats the reason for the WIP - PARALLEL was introduced in 9.6