MB-100: fixed backtrace sanitization to not exclude too much

Change-Id: I43a81485e2a2558a5001007efd3462a984ca73f8
Reviewed-on: http://review.couchbase.org/30109
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>