Support mixed conflated SendChannel with FIFO buffer

[altavir]

I have a problem (device readout), where I can't allow to suspend sender (the device can't wait), and I do not want to allow infinite channel, since listener could never arrive. But on the other hand I want for listener to be able to "catch up" with sender in case it is attached lately or there is a burst of device events. For that it would be helpful to have a FIFO-buffer inside the channel to cache few last values and evict the oldest one, when new value arrives in case the buffer is full.

[elizarov]

Thanks. Noted. It does make total sense with flows, too. When you share flows (see #1716) it would be also helpful to provide fine-grained configuration on how much data is cached for later arriving subscribers. This is somewhat similar to this use-case.