Introduce 'priorToServerVersion' for plugins

Plugins can define a minServerVersion in plugin.xml, which defines the minimum version of Openfire that is required to run the plugin. A similar attribute that defines the maximum version, or a version on or after which a plugin can no longer be used, should be added.

This feature could be used for plugins of which the functionality has been merged into Openfire's core code at some point. When that happens, it no longer makes sense to have the plugin loaded, as that would then offer a duplicate (potentially conflicting) implementation.