Add stats_reset to pg_stat_all_tables|indexes and related views

Hi hackers,

while working on relfilenode statistics [1], I observed that it's possible to call
pg_stat_reset_single_table_counters() on a relation but the reset time is not
reported in the related views.

I think it's interesting to know when the stats have been reset, so the attached
patch is adding stats_reset to the related views.

Also, it's more consistent with other stat kinds that also report reset times.

This new field is not included into the pg_stat_xact_* views because the 0 values
in transaction-local stats have nothing to do with reset operations.

The patch is pretty straightforward, it:

- Adds a reset_timestamp_cb to PGSTAT_KIND_RELATION
- Adds a function to retrieve the stats_reset field (note that this function is
created on top of the existing PG_STAT_GET_RELENTRY_TIMESTAMPTZ macro, so lacking
some flexibility regarding the function name)
- Adds the stats_reset field in the views that are concerned
- Updates the documentation
- Updates some tests

Regards,

[1]: https://postgr.es/m/ZlGYokUIlERemvpB%40ip-10-97-1-34.eu-west-3.compute.internal

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com