Clarify issue of PreviousClock undefined with native store

[martinsumner]

If the PreviousClock is undefined, and the store is native the previous clock is not fetched, and is instead assumed to be the same as the passed clock.

https://github.com/martinsumner/kv_index_tictactree/blob/master/src/aae_controller.erl#L648-L661

There are two scenarios where the previous clock may be undefined:

• On a rehash
• On a fast-write-path put (with no read before write)

The latter part cannot happen in a native store in Riak. This is because read-before-write puts are disabled for a backend with the 2i capability (and IndexBackend).

In either case fetching the previous object clock that doesn't make sense, as the database may have moved on since the aae_put was cast. The clock might now be a future clock, a present clock or a previous clock.

The implemented behaviour is probably the right behaviour - but it should be more obvious and explicit that native mode will not do the potentially expected thing when run in native mode, rather than mysteriously doing something inert.