update go.mod

Merge pull request etcd-io#283 from tklauser/lock-unlock-x-sys-windows

Use `golang.org/x/sys/windows` for `FileLockEx`/`UnlockFileEx`

fix rollback panic bug (etcd-io#153)

remove excessive []byte(s) conversion (etcd-io#143)

`copy` permits using to mix `[]byte` and `string` arguments without
explicit conversion. I removed explicit conversion to make the code simpler.

Signed-off-by: Iskander Sharipov <[email protected]>

Merge pull request etcd-io#65 from zrss/fix-page-cnt

tx: fix the number of pages is incorrectly counted