Buffering iterator

This PR introduces a new function in iterator, iterator.nextBuffering that returns an array for key-value pairs.

When I run the _very_ simple benchmark included I'm seeing some significant speed improvment (2x to 3x faster) by using `nextBuffering` compared to `.next`.

Is this something that we're interested in having in leveldown? And how do we want the API to look like, if so?

There's some stuff missing, like documentation, but I've ported the iterator-tests and they are all passing - but that doesn't mean that there aren't bugs.
Basically I wanted to get this out there so that I can get some feedback if this belongs to node-levedown at all.