xrootd: add open request

Created by: EgorMatirov

Updates #170 (haven't included in the commit message since I expect rebases because of https://github.com/go-hep/hep/pull/214 and https://github.com/go-hep/hep/pull/211 and it will pollute issue with each commit)

Note that open request isn't added to signing.go because it creates a cycle import. Any suggestions on how to deal with that? Probably we can move signing to protocol/signing to fix that?

Also, note that there is a test workaround due to https://github.com/xrootd/xrootd/issues/721.

Also, I'm unsure about UnmarshalXrd in EntryStat and NewEntryStat. It duplicates logic, but otherwise call to UnmarshalXrd in dirlist.go, (Response) UnmarshalXrd(...) will be like that:

o.Entries[i/2].UnmarshalXrd(xrdenc.NewRBuffer([]byte(lines[i+1)]))