Conditionalize LevelDB index retrievals.

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.