Make batch-insert configurable

[kolovos]

In a scenario where the user needs to index many small models, we may want to provide them with the option of switching off batch-insert so that they can trade speed for guaranteed consistency.

[agarciadom]

This might have some unintended consequences for the OrientDB backend, as it requires indexes and node/edge classes to be created outside a transaction. We might end up having to prematurely commit a transaction inserting the first version of a file just to do that.