Provide an option on the JGroups backend for synchronous updates

https://github.com/hibernate/hibernate-search/pull/399

@Hardy Ferentschik CapeDwarf / Blue needs it urgently. It uses Infinispan Query as the storage implementation for the GAE API, and when deployed in clustered configuration they need synchronous consistency levels for some specific entities.

That's also why the synch/asynch option is independent from the Channel, so that the sync flag can be applied only to the specific index needing this.

When I initially opened this issue I flagged it as a bugfix as I was convinced myself it would be synch by default, and I entirely see this useful for other users as well.

We should add an option for who needs to use JGroups with a synchronous backend.

What's the use case? Who needs that?

@Emmanuel Bernard good points I changed it to be a new feature with an independent configuration property. Raising it on the mailing list now.

I disagree. The JMS backend always behaves async in the same vein but there is a difference between sync and async in that you send the message in the same thread or not.

So it makes sense to add an option like wait_for_indexing but that should not be confused witht he sync/async option we have.