If there are any clients connected to the DB, PostgreSQL won't let you
drop the database. It's possible that Sidekiq, Unicorn, or some other
client will be hanging onto a connection, preventing the DROP DATABASE
from working. To workaround this problem, this method cancels all the
connections so that the db:reset command will work.

Note that there's still a slight possibility a client connects after its
connection is terminated. If this is an issue, we could solve it by
revoking CONNECT access, but for now it seems this works.

Closes https://gitlab.com/gitlab-org/gitlab-development-kit/issues/450