pgsql: Introduce the 'force' option for the Drop Database command.

Introduce the 'force' option for the Drop Database command.

This new option terminates the other sessions connected to the target
database and then drop it. To terminate other sessions, the current user
must have desired permissions (same as pg_terminate_backend()). We don't
allow to terminate the sessions if prepared transactions, active logical
replication slots or subscriptions are present in the target database.

Author: Pavel Stehule with changes by me
Reviewed-by: Dilip Kumar, Vignesh C, Ibrar Ahmed, Anthony Nowocien,
Ryan Lambert and Amit Kapila
Discussion: https://postgr.es/m/CAP_rwwmLJJbn70vLOZFpxGw3XD7nLB_7+NKz46H5EOO2k5H7OQ@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/1379fd537f9fc7941c8acff8c879ce3636dbdb77

Modified Files
--------------
doc/src/sgml/ref/drop_database.sgml | 33 +++++++-
src/backend/commands/dbcommands.c | 34 +++++++-
src/backend/nodes/copyfuncs.c | 1 +
src/backend/nodes/equalfuncs.c | 1 +
src/backend/parser/gram.y | 43 +++++++++-
src/backend/storage/ipc/procarray.c | 114 +++++++++++++++++++++++++++
src/backend/tcop/utility.c | 10 +--
src/bin/psql/tab-complete.c | 4 +
src/include/commands/dbcommands.h | 3 +-
src/include/nodes/parsenodes.h | 1 +
src/include/storage/procarray.h | 1 +
src/test/regress/expected/drop_if_exists.out | 10 +++
src/test/regress/sql/drop_if_exists.sql | 7 ++
13 files changed, 248 insertions(+), 14 deletions(-)