Useful system keys should be exposed through the bindings

There are some system keys that probably would be useful to expose through the bindings (perhaps like the management API, see #2960), but also things like `\xff/primaryDatacenter`. There are also probably some that require extra handling, like status JSON or even things that have more "schema" to them, like the client info (i.e., the sampled transactions).

There was some discussion in #2662 about how it would be nice if this could be done automatically through some kind of code generation process, especially if more features are going to be added through this API.