Optionally track memory usage of each LruCache (#9881)

This will double count slightly in the presence of interned strings. It's off by default as it can consume a lot of resources.

synapse/python_dependencies.py

@@ -116,6 +116,8 @@ CONDITIONAL_REQUIREMENTS = {
     # hiredis is not a *strict* dependency, but it makes things much faster.
     # (if it is not installed, we fall back to slow code.)
     "redis": ["txredisapi>=1.4.7", "hiredis"],
+    # Required to use experimental `caches.track_memory_usage` config option.
+    "cache_memory": ["pympler"],
 }
 
 ALL_OPTIONAL_REQUIREMENTS = set()  # type: Set[str]