Avoid race condition between indexing threads

When more than one thread try to index a change in the target instance,
it could be possible that, because of a race condition, the last thread
writes an outdated status to the index.

Synchronize the threads indexing the same change while still allowing
parallelization between threads indexing different changes.

Change-Id: Ic47c9d220f124c096e5e66bb46d13a9a7b0f51e7