in varnish-cache, access to all ESI sub-requests happens in a single
thread, but vmods (VDPs) may add concurrency.

We thus protect access to PRIV_TOP with the session mutex.

Any vmods using this facility will likely need to add additional locking for
the actual data structures referenced through the PRIV_TOP and any other
access to the top request.

For alternatives previously considered, see #3139