*: use TeeReader to reduce I/O in CAS

I tested this patch because it modifies the code where #158 errors. I am getting this as a result of this change:

```
# rkt run https://github.com/coreos/etcd/releases/download/v0.5.0-alpha.4/etcd-v0.5.0-alpha.4-linux-amd64.aci
run: error setting up stage0: error setting up image sha256-701c24b2d275f0e291b807a464ae2390bcd8d7c5b4f2d7e47e6fd917cd5e5588: image hash does not match expected (baed45704bfbab4c99cf98c4a33bab7f1b49808aaf8e5f9cc45aec03e8e83181 != 701c24b2d275f0e291b807a464ae2390bcd8d7c5b4f2d7e47e6fd917cd5e5588)
```