Wait-for-db periodically fails on startup

[hellobrett]

Depending upon the timing of the container startup, the wait-for-db command may produce false positives, i.e., it will think the database is ready when it is not. In fact, I think the current implementation of wait-for-db does not work -- it only gets lucky. The connection object is lazy and must be used in order to produce the OperationalError exception. This PR calls out for a cursor object, which reliably tests that the database is functional.

Steps to Reproduce:

You can reproduce this pretty reliably by repeating the following steps a few times:

docker system prune  #<-- remove all stopped containers
docker-compose up

docker system prune will delete any cached containers (in a stopped state). In this case, the app container will sometimes start before the db container, yielding the error below. Notice in the log trace that the app_1 container is spinning up at the same time as the database and produces a false positive:

app_1  | Waiting for database...
app_1  | Database available!

Full stack trace for false positive:

db_1   | performing post-bootstrap initialization ... sh: locale: not found
db_1   | 2022-02-08 13:28:26.326 UTC [30] WARNING:  no usable system locales were found
db_1   | ok
app_1  | Waiting for database...
app_1  | Database available!
db_1   | syncing data to disk ...
db_1   | WARNING: enabling "trust" authentication for local connections
db_1   | You can change this by editing pg_hba.conf or using the option -A, or
db_1   | --auth-local and --auth-host, the next time you run initdb.
db_1   | ok
db_1   |
db_1   | Success. You can now start the database server using:
db_1   |
db_1   |     pg_ctl -D /var/lib/postgresql/data -l logfile start
db_1   |
db_1   | waiting for server to start....2022-02-08 13:28:27.202 UTC [36] LOG:  listening on Unix socket "/var/run/postgresql/.s.PGSQL.5432"
db_1   | 2022-02-08 13:28:27.215 UTC [37] LOG:  database system was shut down at 2022-02-08 13:28:26 UTC
db_1   | 2022-02-08 13:28:27.219 UTC [36] LOG:  database system is ready to accept connections
db_1   |  done
db_1   | server started
app_1  | Traceback (most recent call last):
app_1  |   File "/usr/local/lib/python3.7/site-packages/django/db/backends/base/base.py", line 216, in ensure_connection
app_1  |     self.connect()
app_1  |   File "/usr/local/lib/python3.7/site-packages/django/db/backends/base/base.py", line 194, in connect
app_1  |     self.connection = self.get_new_connection(conn_params)
app_1  |   File "/usr/local/lib/python3.7/site-packages/django/db/backends/postgresql/base.py", line 178, in get_new_connection
app_1  |     connection = Database.connect(**conn_params)
app_1  |   File "/usr/local/lib/python3.7/site-packages/psycopg2/__init__.py", line 130, in connect
app_1  |     conn = _connect(dsn, connection_factory=connection_factory, **kwasync)
app_1  | psycopg2.OperationalError: connection to server at "db" (172.24.0.2), port 5432 failed: Connection refused
app_1  | 	Is the server running on that host and accepting TCP/IP connections?