Pull database-related code out of the Extractor

Harvested statistics are written to a file on disk using Hypersonic SQL (HSQL), an embedded Java database.  A number of the details of this are exposed to the `Extractor`, which really shouldn't know about _how_ the statistics are written to disk.  So, I moved the HSQL-specific code into a `StatisticsWriter` class which is injected into the `Extractor`.

These changes address TripwireInc/jmxdatamart#3