Add an option to export both past and future events

[IyeOnline]

• The export operators now features a --retro
  flag. This flag will make the operators export past events, even when --live
  is set:

  • Specifying both --retro --live will first export all past events and
    then go into live mode.
  • Specifying only --live behaves like it did previously. It only exports
    new events.
  • Specifying only --retro behaves as if the flag was not set, just like
    previously without any flag.
  • Specifying none of the flags is also unchanged.

  The combination of both flags is experimental. It has an obvious issue with events that happen live while exporting previous events.

• The diagnostics and metrics operators also feature these flags, with the
  same behavior.

• A one batch delay in export --live has been fixed.

[dominiklohmann]

I think this can and should be merged—even with the gap in the data that we currently have between the two modes this still adds quite some value.