"Non-atomic" warning messages on Windows when updating confluence.cfg.xml

Example message:

[atlassian.config.xml.AbstractDom4jXmlConfigurationPersister] saveDocumentAtomically Unable to move D:\confluence\data-2.10.2\confluence.cfg.xml9070676172658464167tmp to 
D:\confluence\data-2.10.2\confluence.cfg.xml. Falling back to non-atomic overwrite.

Atomic file updates seem to be reasonably close to impossible on Windows. We should suppress this warning message (as it's not something the admin can do anything about). Ultimately, we should also fix CONF-8898 to make the update process safer, but that is not covered by this issue.