logbuf: Close the stream after the log is closed.

This simulates old behavior, though I admit there's a possible race. We're
redoing log aggregation in the future anyhow, and I feel that state tracking
shouldn't be a part of logbuf, but that's the best solution I have right now.

Signed-off-by: Blaž Hrastnik <blaz.hrast@gmail.com>