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? 

A type safe schema would be nice, but It doesn't necessarily make life easier, example
```
<directory>
   <filesystem docBase=...>
   </filesystem>
</directory>
<index-reader>
...
</index-reader>
<index-manager>
...
</index-manager>
<worker>
   <backend name="lucene"/>
</worker>
...
```
It can become pretty verbose very quickly :)