Add functions to set up network and database connection(s) separately

[crclark]

As pointed out here, #48 (comment), connecting to multiple databases within a single process is allowed, but setting up multiple network threads is not.

At the moment, we only expose a withFoundationDB function, which sets up the network and connects to the database. Because it does both, it can't be used to connect to multiple databases.

Let's add a separate withNetwork function (with a warning to only call it once) and a separate withDatabase function for advanced use cases when we want to connect to multiple databases.