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 didn't mean to enable indexing by default, but to do an auto-configuration when indexing is enabled.
Gustavo is correct it's tricky to get right considering the overrides, but I'd like to push ourselves hard to really minimize the set of flags needed.
Potentially even to consider a split between "fine tuning" properties, and allow the user to normally only specify a subset of properties, or even to get rid of all properties and use a typesafe schema for all options we want to expose.