Freeze allocated objects on startup. (#6953)

This may make gc go a bit faster as the gc will know things like
caches/data stores etc. are frozen without having to check.

changelog.d/6953.misc

@@ -0,0 +1 @@
+Reduce time spent doing GC by freezing objects on startup.

synapse/app/_base.py

@@ -279,6 +279,15 @@ def handle_sighup(*args, **kwargs):
 
  setup_sentry(hs)
  setup_sdnotify(hs)
+
+ # We now freeze all allocated objects in the hopes that (almost)
+ # everything currently allocated are things that will be used for the
+ # rest of time. Doing so means less work each GC (hopefully).
+ #
+ # This only works on Python 3.7
+ if sys.version_info >= (3, 7):
+ gc.collect()
+ gc.freeze()
  except Exception:
  traceback.print_exc(file=sys.stderr)
  reactor = hs.get_reactor()