Add support for connection multiplexing with DbDataSource

[madelson]

Today, we can't support multiplexing for DbDataSource (only supported in Postgres) because the lock pool is keyed on connection string and also uses connection string as its factory input.

We could extend this to support DbDataSource as a key (reference equality) and then also provide DbDataSource constructors to any other providers that don't have them yet.

[teesofttech]

@madelson , I can do some research about this and, if possible, have a draft PR.

Can you assign it to me?