Implement external database log storage plugin

A/ Execute unit tests for all supported databases B/ Optionally test manually This needs to be tested in all databases. (Note: Your actual Moodle can be in any single database, but this must be tested with all supported databases for external database) Enable external log storage plugin Setup a table in a database outside your regular Moodle database. The table should be identical to the one that is created by standard logstore plugin. Goto the settings for external log store plugin, My home ► Site administration ► Plugins ► Logging ► External database log. Enter your Db config, enter incorrect db password. Make sure an error is generated when you run the 'test connection' tool (after saving). Enter incorrect table name, with rest details accurate. Make sure an error is generated when you run the 'test connection' tool (after saving). Enter all correct details. Make sure all configs are saved and no errors are noticed when your run the 'test connection' tool. Goto any report for example livelogs, and update the drop down to use external database store to display the logs. Do some actions which generate log and make sure those actions are shown in the report. Look at the database and make sure entries are added to the table that you specified. Update filter settings to ignore guest actions. Do some action that generate guest logs, make sure logs are not generated. Exclude levels, teaching and participation and save. Do some action that generate logs with these actions, make sure logs are not generated. Exclude actions, create and delete and save. Do some action that generate logs with these actions, make sure logs are not generated. Repeat tests in MDL-43759 , for external database store, to make sure buffering is working.