pgsql: pg_stat_statements: Track time at which all statistics were last

pg_stat_statements: Track time at which all statistics were last reset.

This commit adds "stats_reset" column into the pg_stat_statements_info
view. This column indicates the time at which all statistics in the
pg_stat_statements view were last reset.

Per discussion, this commit also changes pg_stat_statements_info code
so that "dealloc" column is reset at the same time as "stats_reset" is reset,
i.e., whenever all pg_stat_statements entries are removed, for the sake
of consistency. Previously "dealloc" was reset only when
pg_stat_statements_reset(0, 0, 0) is called and was not reset when
pg_stat_statements_reset() with non-zero value argument discards all
entries. This was confusing.

Author: Naoki Nakamichi, Yuki Seino
Reviewed-by: Yuki Seino, Kyotaro Horiguchi, Li Japin, Fujii Masao
Discussion: https://postgr.es/m/c102cf3180d0ee73c1c5a0f7f8558322@oss.nttdata.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/2e0fedf0362cc964c4dae42258455b6391051e70

Modified Files
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.../pg_stat_statements--1.8--1.9.sql               |  5 ++-
contrib/pg_stat_statements/pg_stat_statements.c    | 46 ++++++++++++++++------
doc/src/sgml/pgstatstatements.sgml                 | 18 +++++++--
3 files changed, 53 insertions(+), 16 deletions(-)