fix some FIXMEs, see #3192

- remove some outdated ones (which were already fixed but not removed)
- make socket options fully configurable in NettyTransport
- also apply socket options to outbound connections, not only the listen
  socket
- remove timeout from taking the lock in RepointableActorRef, because we
  cannot afford to lose system messages anymore, so we should rather
  block the sender
BackpressureSpec was not testing what I thought it did