logging: Log only for unhandled errors, remove all the debug logging. (#1652)

This patch brings in the removal of debug logging altogether, instead
we bring in the functionality of being able to trace the errors properly
pointing back to the origination of the problem.

To enable tracing you need to enable "MINIO_TRACE" set to "1" or "true"
environment variable which would print back traces whenever there is an
error which is unhandled or at the handler layer.

By default this tracing is turned off and only user level logging is
provided.

auth-handler.go

@@ -111,12 +111,10 @@ func sumMD5(data []byte) []byte {
 // Verify if request has valid AWS Signature Version '4'.
 func isReqAuthenticated(r *http.Request) (s3Error APIErrorCode) {
 	if r == nil {
-		errorIf(errInvalidArgument, "HTTP request cannot be empty.", nil)
 		return ErrInternalError
 	}
 	payload, err := ioutil.ReadAll(r.Body)
 	if err != nil {
-		errorIf(err, "Unable to read HTTP body.", nil)
 		return ErrInternalError
 	}
 	// Verify Content-Md5, if payload is set.