AiiDA opening two connections per daemon instance

[muhrin]

This issue may apply beyond just the daemon but this is where it's easiest to see.

To reproduce:

1. Log in a psql shell and run SELECT sum(numbackends) FROM pg_stat_database;, note the number
2. verdi devel run_daemon
3. Check SELECT sum(numbackends) FROM pg_stat_database; again. It is likely that there will be 2.

This seems to happen with both Django and SQLA and contrary to my initial thoughts it's not because of the 'new' kiwipy communicator which has a seperate thread.

The second connection (at least in SQLA) seems to be opened when (or around) this line is called: https://github.com/aiidateam/aiida_core/blob/ebf16441a3910d5b783100c13ddf3b5f6fcc6b1f/aiida/backends/sqlalchemy/globalsettings.py#L53

Unfortunately because of the connection pool this new connection will remain open. One way to get around this is to set the pool size to 1 in create_engine but I don't know how safe this is...

[sphuber]

This seems to no longer be a problem. Tested this after #5330 was merged, at which point only SqlAlchemy is still supported and I only see a single connection to postgres when running verdi devel run_daemon.