Refactor database handling and add additional support

Per #21 , now that read-only connections are preferred (to stop locks from console usage #18 ), this should be supported for MySQL and sqlite3.  Database type handling needs to be addressed, and the TODOs from the diverging development vs. CI build environments need to be resolved.  https://github.com/salsify/safer_rails_console/blob/master/spec/integration/patches/sandbox_spec.rb