Add last_vacuum_index_scans in pg_stat_all_tables

Hi hackers,

I think having number of index scans of the last vacuum in
pg_stat_all_tables can be helpful. This value shows how efficiently
vacuums have performed and can be an indicator to increase
maintenance_work_mem.

It was proposed previously[1], but it was not accepted due to the
limitation of stats collector. Statistics are now stored in shared
memory, so we got more rooms to store statistics. I think this
statistics is still valuable for some people, so I am proposing this
again.

Best wishes,

--
Ken Kato
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION

[1]
https://www.postgresql.org/message-id/20171010.192616.108347483.horiguchi.kyotaro%40lab.ntt.co.jp