Validate diskFrontier domain for series candidate.

It is the case with the benchmark tool that we thought that we
generated multiple series and saved them to the disk as such, when
in reality, we overwrote the fields of the outgoing metrics via
Go map reference behavior.  This was accidental.  In the course of
diagnosing this, a few errors were found:

1. ``newSeriesFrontier`` should check to see if the candidate fingerprint is within the given domain of the ``diskFrontier``.  If not, as the contract in the docstring stipulates, a ``nil`` ``seriesFrontier`` should be emitted.

2. In the interests of aiding debugging, the raw LevelDB ``levigoIterator`` type now includes a helpful forensics ``String()`` method.

This work produced additional cleanups:

1. ``Close() error`` with the storage stack is technically incorrect, since nowhere in the bowels of it does an error actually occur.  The interface has been simplified to remove this for now.
Merge pull request #115 from prometheus/fix/storage/nil-behaviors

Validate diskFrontier domain for series candidate.