Otherwise there might still some thread waiting for the receiver to
consume a message and we would deadlock here. Dropping the receiver
drops all messages and unblocks them all immediately.
Also don't panic when sending messages to a dropped receiver.
The only None ranks now are:
- cdgdec/cdgparse: Primary
- rav1enc: Primary
- reqwesthttpsrc: Marginal
- s3sink/s3src: Primary
- mccenc/mccparse: Primary
- sccenc/sccparse: Primary
All other ranks are set to None until the elements are ready to replace
the existing C elements, or are otherwise production-ready and of a
element kind that should be autoplugged in one way or another.
For example the sodium elements are production-ready but can't be
autoplugged and as such should stay at a None rank.
