Do not assume numbers for comparisons in top.

This allows hierarchical sorting, useful to determine trends.

For instance:

```clojure
(top 10 (juxt :metric :time)
     (tag "spiking" index)
     index)
```
Correct switch to compare instead of number assumptions.
Provide a demotion stream for events bumped out of the top-k.
Merge pull request #306 from pyr/feature-top-expire

Do not assume numbers for comparisons in top.