Allow replication slots to be dropped in single-user mode

Allow replication slots to be dropped in single-user mode

Starting with commit 9915de6c1cb2, replication slot drop uses a
condition variable sleep to wait until the current user of the slot goes
away.  This is more user friendly than the previous behavior of erroring
out if the slot is in use, but it fails with a not-for-user-consumption
error message in single-user mode; plus, if you're using single-user
mode because you don't want to start the server in the regular mode
(say, disk is full and WAL won't recycle because of the slot), it's
inconvenient.

Fix by skipping the cond variable sleep in single-user mode, since
there can't be anybody to wait for anyway.

Reported-by: tushar <tushar.ahuja@enterprisedb.com>
Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://postgr.es/m/3b2f809f-326c-38dd-7a9e-897f957a4eb1@enterprisedb.com