stage0: use TeeReader instead of MultiWriter
cas: eliminate a read/write from WriteACI path
*: split out LimitedWriter into pkg
stage0: ensure entire file is copied into hash
pkg: rename LimitedWriter -> LimitedReadWriter
pkg: LimitedReadWriter -> LimitedWriter
Merge pull request #154 from jonboulle/master

*: use TeeReader to reduce I/O in CAS