Bulk on single thread and other bulk related stuffs refactorings

In current master branch, bulk updates are handled using an additional Thread, in order to transform the Java beans into a JSON stream, without using to much memory.

I implemented an alternative way of transforming a Collection of beans into a JSON stream, without relying on the usage of a separated thread.
The streaming is done using an implementation of HttpEntity from Apache HttpClient.

This PR adds the ability to use that alternative.

The default implementation is still the multi-threaded way.

But it is easy for any user to switch to the single threaded way, using dependency injection.