RU: stop RF after all threads finished
If we do not stop RF after all threads, it can happen that the RF is finished, but another thread of the RU (e.g., reorder thread) tries to send out a package to the RF. This can lead to a segfault, as observed in UHD, because we free all resources before reusing them. This frequently lead to failures in the CI (in short, the CI did its job).
Another commit reorders condition signal before mutex unlock.