Indexing configured via XML should be automatically enabled if the 'enabled' attribute is missing

The 'enabled' attribute has a default of 'false' in xsd, which is fine for all purposes when indexing is not configured for a cache. Indexing should generally not be enabled by default as it is costly, unless the <indexing> element is present and the 'enabled' attributive was omitted (and if the a tribute was not omitted, then we do what the attribute says).

Also, this auto-enabling based on <indexing> element presence should only be performed since schema ver. 11.

Programmatic config is not going to be affected by this; `enable()` will have to be called explicitly.