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? 

I though the autoconfig initially as a builder time construct only (sort of a hint), but I will propagate it to the immutable counterpart, allowing to detect at runtime if the configuration was generated or not