now created through the index itself
mapping problems reported early at the time
data is added to the batch, previously these
were not reported until the batch was executed
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.