gh-160549: Fix crash when freeing a busy multi-threaded ZstdCompressor (GH-160550)

ZSTD_freeCCtx() frees a dictionary loaded with ZSTD_CCtx_loadDictionary()
before it stops the worker threads.  Make libzstd wait for unfinished
jobs before freeing the compression context of a ZstdCompressor that
uses worker threads and a ZstdDict.

Works around https://github.com/facebook/zstd/issues/4862