Concurrent entities processing issues in the Search Add-on

[fractal3000]

There are two possible problems with concurrency in the Search Add-on:

1. There is possibility in case of parralel execution of two operations(index configurations updating and indexing of entities) the wrong index configuration will be used. The proposal is to block indexing operation for the duration of the index configurations sinchronization process
2. It is possible that during the single operation searching and (especially) indexing data the two diferent configurations of the single index will be used. The proposal is to check such cases and correct the behavior.