use AV worker items infrastructure for GIN pending list's cleanup

Hi,

When AV worker items where introduced 4 years ago, i was suggested that
it could be used for other things like cleaning the pending list of GIN
index when it reaches gin_pending_list_limit instead of making user
visible operation pay the price.

That never happened though. So, here is a little patch for that.

Should I add an entry for this on next commitfest?

-- 
Jaime Casanova
Director de Servicios Profesionales
SystemGuards - Consultores de PostgreSQL