Make prelaunch / boot logging more synchronous

[lukebakken]

Currently, the rabbit_prelaunch debug logging does not log messages "as they happen". While working on #2656 this confused me quite a bit as it seemed as though boot functions weren't completing when they should have been. I worked around this by inserting my own logging like this:

io:format(standard_error, "@@@@@@@@ MY OBVIOUS MESSAGE~n", [])

Ideally, during RabbitMQ bootup, debug messages would be logged and flushed ASAP.

[dumbbell]

Could you please describe how to reproduce the problem? I don't see any delay locally.

[lukebakken]

I will attach a startup log today with and without my added standard_error messages. I didn't do anything special to reproduce the issue. It's just that log messages appeared in the console "out of order" which made it appear as though prelaunch and other boot steps were happening at times that didn't make any sense!

[HoloRin]

I will close this since the issue does not appear to have come up again.