Use maxConsecutiveEmptyReads const instead of hardcoded
100 in scan.go too.

Change-Id: I993f353a3748f0d6bdefab38bf5cb224eea8a969
Reviewed-on: https://go-review.googlesource.com/46915Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>