Support database connection pooling

[lobis]

We deploy PgBouncer as part of all our Rucio deployments with good results.

After some other users have reported issues with the number of db connections, I think this feature could become a part of the upstream charts (opt-in).

In our setup we have all db connections go through the pool but I can see this would not be as easy to implement here because there is not a unified chart. However I think adding the pool at the daemons chart would be enough to provide a significant improvement since here is where most connections are created.