ISPN-4340 Automatically setup shared indexes when indexing is enabled

For review only

https://issues.jboss.org/browse/ISPN-4340

@Sanne, @anistor
Those extra properties add a significant performance gain without too much tinkering with hibernate-search properties, at the expense of an extra attribute in the config. Thoughts? 

That was how the initial implementation looked like. But imagine if the user manually adds 1 property only,  at runtime the index would be configured with 1 plus all the auto configured ones. IMHO not very transparent if one tries to figure out how the index is configured
 
By making autoconfig usage explicit, at least there's a consent (also documented) on which properties are being added. WDYT?