pgsql: Extend pg_stat_statements_reset to reset statistics specific to

Extend pg_stat_statements_reset to reset statistics specific to a
particular user/db/query.

The function pg_stat_statements_reset() is extended to accept userid, dbid,
and queryid as input parameters. Now, it can discard the statistics
gathered so far by pg_stat_statements corresponding to the specified
userid, dbid, and queryid. If no parameter is specified or all the
specified parameters have default value aka 0, it will discard all
statistics as per the old behavior.

The new behavior is useful to get the fresh statistics for a specific
user/database/query without resetting all the existing statistics.

Author: Haribabu Kommi, with few additional changes by me
Reviewed-by: Michael Paquier, Amit Kapila and Fujii Masao
Discussion: https://postgr.es/m/CAJrrPGcyh-gkFswyc6C661K6cknL0XkNqVT0sQt2mFNMR4HRKA@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/43cbedab8ff1eef4088807ffc1a64a107de67af6

Modified Files
--------------
contrib/pg_stat_statements/Makefile | 8 +-
.../expected/pg_stat_statements.out | 198 +++++++++++++++++++++
.../pg_stat_statements--1.6--1.7.sql | 22 +++
contrib/pg_stat_statements/pg_stat_statements.c | 96 ++++++++--
.../pg_stat_statements/pg_stat_statements.control | 2 +-
.../pg_stat_statements/sql/pg_stat_statements.sql | 64 +++++++
doc/src/sgml/pgstatstatements.sgml | 106 ++++++++---
7 files changed, 455 insertions(+), 41 deletions(-)