[cbcollect_info] flush all output on line-by-line basis

For better chance to diagnose MB-11356

Change-Id: I7f261f4f0a8fd0ace267db1abd06a11b542e8693
Reviewed-on: http://review.couchbase.org/38055
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>