XTR: stats export fails with MySql server gone away

[dasgarner]

After exporting a set of stats, we refresh/reconnect to MySQL and create a media file and upload the exported CSV to the library for storage. When this media file is saved we call audit to create an audit record for that.

Audit logging uses a prepared SQL statement, which is attached to the old MySQL connection and therefore fails.

There are three options:

1. disable auditing for that operation
2. add a method to the logging service to clear cached PDO statements and call this before auditing
3. implement a retry loop in audit logging which does a reconnect

Option number 3 is the most isolated change, although does mean an exception handler.

This also effects v3.