LogAnalyzer - DataflashSQL explore existing databases

 - move metadata and all mapping related things to DataflashSQL  instances
   avoids remapping already mapped classes when opening a second database
 - learn database format from database if fmt table exists
   this is even better than SQLAlchemy automap, as we can have the same
   inheritance as when converting logs to db, it is even the same code
 - remove Import/Explorer, DataflashSQL is a Explorer by default,
   and convert() can be used to import data from logfiles