Instead of using SetOutput(ioutil.Discard), which means
that Printf is still called for every debug message,
use a simple and fast boolean check.
Streaming write performance improves from 86 to 93 MB/s.
Use that option to speed up the automated tests by 7 seconds.
Before:
ok github.com/rfjakob/gocryptfs/integration_tests 26.667s
After:
ok github.com/rfjakob/gocryptfs/integration_tests 19.534s
