• moving session cleanup to pre_close hook to avoid races with
  modules already shut down and slave connections still operating.
• stream timeouts now change to vhost values once the request
  is parsed and processing starts. Initial values are taken from base
  server or SNI host as before.
• fixed retry behaviour for http2 proxy connections when frontend
  connection uses http/1.1.
• separate mutex instances for each bucket beam, resulting in
  less lock contention. input beams only created when necessary.