Inherit dbtables monitoring 2

By these patches, DBTablesMonitoring becomes a subclass of DBTables.

Since DBHatohol is an owner of DBTablesMonitoring and uses DBAgentMySQL, DBTablesMonitoring also uses DBAgentMySQL. Althogh I ran this version and confirmed the behavior actually, it may be potential bugs due to this big change of the backend DB. 
Although I already merged it, this message seems unfriendly.
I think "Unknown DB type" or something is more friendly.