Add a way to have receivers that only care about the latest received value

[llucax]

This means not logging when the buffer is full.

We have a proposal:

• Support for disabling overflow-warning logs in broadcast receivers #329

My feeling is this use case is special enough that it probably deserves a special receiver. Broadcast channel receivers are actually buffered, and if we want to receive only the latest value, that means that's unbuffered, so one option would be to add a new_unbuffered_receiver() that doesn't take a limit argument at all.

[Marenz]

That suggestion sounds fine to me