Added support for async transaction callbacks in InMem DB and API.
Fixed async HTTP handling mechanism.
Removed save-to-output-stream method from the DB API.
Renamed "load" and "save" methods in InMem DB.
Refactored Rapidoid CSS and JS out into separate files.
Improved readability of error message arguments.
Added support for HttpExchange argument in non-generic command handlers.
Removed the obsolete "responseCode" method in HttpExchange.
Added response code getter in HttpExchange.
Added support for redirect on AJAX events.
Implemented tracking of the previous page in session.