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? 

TBH, I can't make any sense of this :)