Re: Add index scan progress to pg_stat_progress_vacuum

Attached is v13-0001--Show-progress-for-index-vacuums.patch which addresses
the latest comments. The main changes are:

1/ Call the parallel_vacuum_progress_report inside the AMs rather than vacuum_delay_point.

2/ A Boolean when set to True in vacuumparallel.c will be used to determine if calling
parallel_vacuum_progress_report is necessary.

3/ Removed global varilable from vacuumparallel.c

4/ Went back to calling parallel_vacuum_progress_report inside 
WaitForParallelWorkersToFinish to cover the case when a 
leader is waiting for parallel workers to finish.

5/ I did not see a need to only report progress after 1GB as it's a fairly cheap call to update
progress.

6/ v1-0001-Function-to-return-currently-vacuumed-or-cleaned-ind.patch is a separate patch
for exposing the index relid being vacuumed by a backend. 

Thanks

Sami Imseih
Amazon Web Services (AWS)