Mutex TTL Cache accesses which can be accessed by multiple threads

Apparently, cachetools is (intentionally) not threadsafe
when using the Caches directly.
It's therefore recommended to wrap these with an explicit lock to avoid
problems.

source: https://github.com/tkem/cachetools/issues/245

closes #7215

freqtrade/optimize/hyperopt.py

@@ -483,6 +483,7 @@ class Hyperopt:
         self.backtesting.exchange._api_async = None
         self.backtesting.exchange.loop = None  # type: ignore
         self.backtesting.exchange._loop_lock = None  # type: ignore
+        self.backtesting.exchange._cache_lock = None  # type: ignore
         # self.backtesting.exchange = None  # type: ignore
         self.backtesting.pairlists = None  # type: ignore