Drop database users prior to DROP/CREATE database.

This fixes an annoying issue where one tries to rebuild the database,
and it fails due to there being existing connections.

The one thing that is potentially scary about this implementation is
that it means it's now a lot easier to accidentally drop your
production database by running the wrong script; might be worth adding
a "--force" flag controlling this behavior or something.

Thanks to Nemanja Stanarevic and Neeraj Wahi for prototypes of this
implementation!  They did most of the work and testing for this.

scripts/setup/postgres-init-db

@@ -1,8 +1,14 @@
-#!/bin/sh -xe
+#!/bin/bash
+set -e
+set -x
 
 # Make sure the current working directory is readable
 cd /
 
+# Drop any open connections to any old database.  Hackishly call using
+# source because postgres user can't read /root/zulip/scripts/setup.
+source "$(dirname "$0")/terminate-psql-sessions" postgres zulip zulip_base
+
 su postgres -c psql <<EOF
 CREATE USER zulip;
 ALTER ROLE zulip SET search_path TO zulip,public;
@@ -15,6 +21,7 @@ CREATE SCHEMA zulip AUTHORIZATION zulip;
 CREATE EXTENSION tsearch_extras SCHEMA zulip;
 EOF
 
+# Clear memcached to avoid contamination from previous database state
 sh "$(dirname "$0")/flush-memcached"
 
 echo "Database created"