Redis redis-cli: Flush output in pubsub mode

We already flush the output in MONITOR mode, which makes it easy to use usual *nix pipes and combine it with sed, grep, etc.

We don't do this for pubsub mode, which results in redis-cli subscribe foo | grep pattern not working as expected (same thing is true for slave mode)

Should we just flush all the time? Or just set output to be line buffered anyway with

setvbuf(stdout, NULL, _IOLBF, 0);

Comment From: yoav-steinberg

Hanlded in #9013