Add scanner over multiple Spans

The original intent of #626 was to support fetching multiple Spans. This was not done because the result may not fit into memory. At the time when #626 was worked on the Scanner API was in a bad state. After the scanner the scanner API improvements in #639, it would be relatively easy to add multiple spans to the API. Could support something like the following.

``` java
Snapshot snap = ...
snap.scanner().over(span1, span2, span3).build();

Collection spans = ...
snap.scanner().over(spans).fetch(col1, col2).build()
```

Multiple spans may result in out of order key iteration.