xlog logging is kind of slow - usually it comes once per several seconds
or once per minute. And without flushing many entries can remain sitting
up in the file buffer in userspace without being conveyed to OS kernel.
Which is not very convenient because in such situation we cannot make good
use of tools like `tail -f`.

Since flushing is relatively cheap operation - it is just one write
syscall - let's do it after every emitted line. The write syscall does
not force data to be synced to disk, so it should not slow things down,
but make it convenient to have latest logs right away in the filesystem
view.