Review the need of semaphores in blobs backend

We've seen problems with having too many connections to the blobs database (#8945 (closed)). Current tests that touch blobs and concurrency all use semaphores to prevent too meny connections, but that is done from user perspective, outside of BlobManager.

We have to check whether the concurrecy problems still exist if we don't use semaphores from outside, and act accordingly (either removing the semaphores from the tests if they are not needed or moving the semaphores inside the BlobManager if they are needed).