client: check type of 'oid' parameter when loading an object

Since the switch to msgpack, there's no more type checking at protocol level
and for example passing an integer would cause storage nodes to crash.

But as shown here, the type checking of the old protocol was not always
enough, because data structures at client side could anyway get wrong.

neo/client/app.py

@@ -411,6 +411,8 @@ class Application(ThreadedApplication):
                         result = self._cache.load(oid, before_tid)
                     if result:
                         return result
+                    u64(oid) # check type to not pollute _loading
+                             #  or crash storage nodes
                     load_lock = self._loading[oid][0]
                     acquired = load_lock.acquire(0)
                 # Several concurrent cache misses for the same oid are probably