[levelup] Instrumentation

This is basic instrumentation for the levelup module.

It includes experimental instrumentation for the batch command. It currently replaces KVKey with KVKeys and KVOps, which are simply arrays of strings. It can also be used as a [chainable API](https://github.com/rvagg/node-levelup#dbbatch-chained-form). The chainable mode doesn't currently work. Also, buffer values probably don't work either yet.

Does the KVKeys/KVOps approach seem reasonable?

You can also use read/write streams with levelup, which I haven't looked too deeply at yet. That looks like a more complicated thing and I'm not sure exactly how we'd want to represent it, or if we even *want* to instrument it.