The LevelDB storage types return an interface type now that wraps
around the underlying iterator. This both enhances testability but
improves upon, in my opinion, the interface design for the LevelDB
iterator.
Secondarily, the resource reaping behaviors for the LevelDB iterators
have been improved by dropping the externalized io.Closer object.
Finally, the iterator provisioning methods provide the option for
indicating whether one wants a snapshotted iterator or not.
The LevelDB index retrievals could be repeated in a given operation
batch if multiple queued mutations affect the same (Label Name) singles
and (Label Name, Label Value) doubles. This is wasteful and
inefficient, as a single retrieval suffices. Thusly this commit
retrieves the canonical index mappings if the said mapping has not
been looked up in a given batch.
Matt T. Proud
Julius Volz