Re-index multiple views together

When updating a view's index, also update other views' indexes. This
saves time because a lot of the work (iterating through the sequences,
fetching the documents, looking for conflicts) is shared.

Like CouchDB, I'm grouping views together for this purpose. The other
views that will get updated are the ones sharing the same prefix up
through the first "/". Or if the view name has no "/", it's all the
other views without a "/".
Optimization for naughty map functions that emit 'doc' as value

Instead of storing the whole doc dictionary, store a placeholder ("*").
At query time, detect the placeholder and load the revision body.
A few more view-indexing optimizations
Changed indexing behavior so views with no “/“ in name don’t index together.

There’s a compile-time flag GROUP_VIEWS_BY_DEFAULT that can be defined
to change this back to having all those views form one group.