now you can access the underlying index/store implementations
using the Advanced() method. this is intedned for advanced
usage only, and can lead to problems if misused.
also, there is a new method NewUsing(...) which allows callers
of the top-level API to choose which underlying k/v store they
want to use.
more things can return error now
in a couple of places we had to swallow errors because they didn't
fit the existing API. in these case and proactively in a few
others we now return error as well.
also the batch API has been updated to allow performing
set/delete internal within the batch
an IndexAlias allows you easily work with one logical Index
while changing the actual Index its pointing to behind the scenes
Changing which actual Index is backing an IndexAlias can be done
atomically so that your application smoothly transitions from
one Index to another.
A separate use of IndexAlias is allowed when the IndexAlias is
defined to point to multiple Indexes. In this case only the
Search() operation is supported, but the Search will be run
on each of the underlying indexes in parallel, and the results
will be merged.
