Keep plugin logs and errors in a system table

[pauldix]

Plugin results return log lines and errors, which we should keep in a system table named plugin_logs that users can query. It should be a circular buffer with a limit like the query log, and others of that kind. We'll need the plugin_name, trigger_name, level, and message. The test endpoints shouldn't populate this table since they return a response to the user with this information right away.