Make config cache thread safe

cachetools cache is not thread safe and there were frequent exceptions
logged indicating that cache updates during async calls were failing
with key errors similar to those described in:

tkem/cachetools#80

Add a lock per table instance synchronizes cache updates across threads
in.

pccommon/pccommon/tables.py

@@ -1,3 +1,4 @@
+from threading import Lock
 from typing import (
     Any,
     Callable,
@@ -58,6 +59,7 @@ def __init__(
         self._service_client: Optional[TableServiceClient] = None
         self._table_client: Optional[TableClient] = None
         self._cache: Cache = TTLCache(maxsize=1024, ttl=ttl or DEFAULT_TTL)
+        self._cache_lock: Lock = Lock()
 
     def _ensure_table_client(self) -> None:
         if not self._table_client:
@@ -187,7 +189,7 @@ def update(self, partition_key: str, row_key: str, entity: M) -> None:
                 }
             )
 
-    @cachedmethod(cache=lambda self: self._cache)
+    @cachedmethod(cache=lambda self: self._cache, lock=lambda self: self._cache_lock)
     def get(self, partition_key: str, row_key: str) -> Optional[M]:
         with self as table_client:
             try: