Wrap LevelDB iterator operations behind interface.

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.
Merge pull request #96 from prometheus/refactor/persistence/iterator-interface

Serious Cleaning of LevelDB Iterator Code