Disable "compression_parallel_threads" in crash test for now (#6816)

Summary:
"compressio_parallel_threads" caused several test failure tests. To keep crash test clean, disable it for now.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6816

Test Plan: "make crash_test" to make sure the python script doesn't break

Reviewed By: zhichao-cao

Differential Revision: D21462112

fbshipit-source-id: 9eecc764800da82cd19665dc8b167eacead3310b

tools/db_crashtest.py

@@ -44,7 +44,9 @@ default_params = {
     "checksum_type" : lambda: random.choice(["kCRC32c", "kxxHash", "kxxHash64"]),
     "compression_max_dict_bytes": lambda: 16384 * random.randint(0, 1),
     "compression_zstd_max_train_bytes": lambda: 65536 * random.randint(0, 1),
-    "compression_parallel_threads": lambda: random.choice([1] * 9 + [4]),
+    # Disabled compression_parallel_threads as the feature is not stable
+    # lambda: random.choice([1] * 9 + [4])
+    "compression_parallel_threads": 1,
     "clear_column_family_one_in": 0,
     "compact_files_one_in": 1000000,
     "compact_range_one_in": 1000000,