frontend, backend: add FrontendClient.{get,post,put}
    
    We shouldn't use FrontendClient's private/protected methods outside of
    frontend.py, same as we shouldn't use manual 'requests.*' methods when
    we have reliable 'frontend_client'.  So enhance FrontendClient API and
    use it everywhere where we talk to `backend_ns`.
    
    Frontend's '/backend/chroots-prunerepo-status/' is naturally GET only,
    so change the methods= argument there (backend will use GET as well
    now).
    
    Merges: #1021