Persistent cache: allow to configure the add data concurrency

During operations that creates and reads a large number of nodes (eg. indexing content) it may happen that there's more items in the asynchronous queue (introduced in OAK-2761) than the queue consumer can handle. As a result, the queue is purged and items are not saved in the cache, which makes the overall performance worse.

An easy fix is to add a property that allows to switch between async/sync mode. By default, it should be synchronous.