Make GIN's cleanup pending list process interruptable

 Make GIN's cleanup pending list process interruptable
 
 Cleanup process could be called by ordinary insert/update and could take a lot
 of time. Add vacuum_delay_point() to make this process interruptable. Under
 vacuum this call will also throttle a vacuum process to decrease system load,
 called from insert/update it will not throttle, and that reduces a latency.
 
 Backpatch for all supported branches.
 
 Jeff Janes <jeff.janes@gmail.com>