revised version of the database backed stateFlowGraph implementation

I was wondering if you could consider this pull request in which all the feedback from the [previous pull request](https://github.com/crawljax/crawljax/pull/295#issuecomment-20498410) has been applied.

please see the comments [here](https://github.com/crawljax/crawljax/pull/295#issuecomment-20495188) and [here](https://github.com/crawljax/crawljax/pull/295#issuecomment-20498410) for more information.