Simplified truncation in _view/bbox.
Reduced amount of pre-filter keys for the 2nd reduction.

At the 2nd reduction neither time nor types are carried into options
necessarily, so the pre-filter has to look the missing one up in the filter
chain.
Now, because I've allowed to branch into pre-filtering only for the 1st and 2nd
filter, the actual look-up-process just has to go through filters[0] and [1]
and this is why it looks as simple as that. My assumption has always been that the
amount of documents must have reduced sufficiently after that.
Improved _view/types-year to use 'sensible granularity'.

Previous tests showed that still way too many documents have to run by the
post-filter because using no range as key if doc.time spans over more than one
year just isn't restrictive enough.
So I've come up with a smarter scheme that is still kind of intuitive:
- Extract the very beginning and the very end from doc.time,
- strip everything except for the year,
- emit range if its a single year or a one-sided range, or
- round to the next five years if doc.time spans over five years or less, and
  ditto for ten, fifty and a hundred years. For instance, "1960-1992" spans
  over 32 years, so a simplified range is "1950-2000".
At the current data set, I end up with 48 unique keys of 3174 in total, and
awesome performance, so job well done I guess.
Turns out I can't do anything (fixed a937bd03f).

The worst part is that this is now a revert of a revert.
Fixed uncaught exception thrown in filter_documents.

This happens when spatial filters clean up the filter queue by using {union:[]}
as the first argument. This causes all filters to be collapsed and the
collapsed filter runs by _list/tabulate. If it contains either time or type,
pre-filtering is used (as it still should), but it stacks {redo:filter} on top
of the argument list. filter_documents then gets confused because it processes
{spatial:relation} before {redo:filter}.
Fixed pre-filter ignoring stored document sets.

These documents have a keep_me tag but the pre-filter ignored it when no redo
was needed.
Renamed _view/types-year to type-years.

It is because each doc emits just one type, but a range of years but what do I
know.