Paginated metrics

Implements #73.

Dug into this to start addressing my problems w/ metric loads in my prod environment, which has something like 800k distinct metric paths and growing.

Right now, with the "load all metrics on start" behavior, I suck down 80MB of metric path data, which kinda sucks when talking to a non-local-to-workstation server:

![](http://note.io/O1JK09)

With pagination enabled & the app set to default to 1 page at a time, it's a much more reasonable 4KB:

![](http://note.io/1j984Y6)

This implementation doesn't appear to affect search results (and no attempt was made to paginate them from the backend, as they're typically much smaller, & I will have another PR soon for making the search itself faster anyways.)