DBAppender redesign [was originally: Customizing table names used by DbAppender]

ch.qos.logback.classic.db.DBAppender uses hard-coded table names for logging events: logging_event, logging_event_property and logging_event_exception. Allowing to change these default names has two benefits:

• There are systems where database table names must follow some naming conventions (prefixes, using underscores vs. camel case)
• One could create several Db appenders, each operating on different set of tables (for performance reasons or more fine grained logging and filtering)

Each table name would be configured using a property in logback.xml. Default values for table names would be the same as present ones to maintain backward compatibility. If you are interested in this feature let me know, I can implement it and provide a patch.