Serious Cleaning of LevelDB Iterator Code

@juliusv, this change depends upon https://github.com/prometheus/prometheus/pull/94, which is presently open for review again @grobie.  If the dependency change is OK with him, I will continue with this pull request here.

Overall, this pull is designed to simplify the code and assumptions around our use of LevelDB iterators as well as increase testability.  Please let me know what you think.

/CC: @grobie and @discordianfish