Code review comment for lp:~akopytov/percona-xtrabackup/parallel-compression

We should probably also add xbstream file format to documentation and make it a bit clearer for anybody who may want to interact with it....

so, my thoughts on merging it are that I think we're pretty good to go, although it may be a good idea that we do any format changes (suggested above) before we merge it in.

we will need branch based on current trunk though, and jenkins run.