common, rpmbuild: repeatedly send requests to frontend until they succeed
    
    Fix #1033
    
    IMHO it doesn't sound like a good idea to duplicate the backend code
    for sending requests repeatedly. This is an ideal piece for the
    `python-copr-common` package.
    
    However, I didn't want to move the whole `FrontendClient` there
    because:
    
    - It's too tied with backend (values from `opts`, methods such as
    `starting_build`, `reschedule_build`, etc). We would have to move a
    part of the `FrontendClient` to the common package and then create a
    class in backend code, that would extend it and add those methods.
    - Its interface would not fit the `copr-rpmbuild` needs. Mainly I am
    talking about passing just `url_path` to a `FrontendClient`, while
    `copr-rpmbuild` works with the whole URL.
    
    I decided to rip the central pieces from `FrontendClient` and create a
    `SafeRequest` class, which can be easily used anywhere. See the change
    in `copr-rpmbuild`, that's just all of it. Additionally, we can utilize
    it in `providers/custom.py` and `providers/spec.py` but this is not
    what this issue is about, so I didn't do it yet.