Honor passed-in database OIDs in pgstat_database.c

Honor passed-in database OIDs in pgstat_database.c

Three routines in pgstat_database.c incorrectly ignore the database OID
provided by their caller, using MyDatabaseId instead:
- pgstat_report_connect()
- pgstat_report_disconnect()
- pgstat_reset_database_timestamp()

The first two functions, for connection and disconnection, each have a
single caller that already passes MyDatabaseId.  This was harmless,
still incorrect.

The timestamp reset function also has a single caller, but in this case
the issue has a real impact: it fails to reset the timestamp for the
shared-database entry (datid=0) when operating on shared objects.  This
situation can occur, for example, when resetting counters for shared
relations via pg_stat_reset_single_table_counters().

There is currently one test in the tree that checks the reset of a
shared relation, for pg_shdescription, we rely on it to check what is
stored in pg_stat_database.  As stats_reset may be NULL, two resets are
done to provide a baseline for comparison.

Author: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Dapeng Wang <wangdp20191008@gmail.com>
Discussion: https://postgr.es/m/ABBD5026-506F-4006-A569-28F72C188693@gmail.com
Backpatch-through: 15