Use-after-free when changing a multithreaded context with an unfinished frame

When a multithreaded frame (zstdmt feature, CParameter::NbWorkers) is left unfinished, its jobs may still be running on the worker threads: the C library only waits for them when the next frame starts, or when the context is freed. A session reset makes the context accept changes again in the meantime, so:

• unloading the dictionary (load_dictionary, ref_cdict, ref_prefix, disable_dictionary, init*, or reset with parameters) freed the dictionary a worker was still compressing against;
• changing NbWorkers made the next frame resize the job table before waiting for the old jobs.

The flaw was corrected by refusing these calls while a multithreaded frame is unfinished. The new CCtx::wait_for_workers() waits for the workers first.