profiling: add streaming read profiling helper
profiling: add streaming read profiling helper

Reads 1GB of zeros while collecting memory and cpu profiles.

rfjakob/gocryptfs,davesteele/gocryptfs,rfjakob/gocryptfs,davesteele/gocryptfs,davesteele/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs