improve http request handling for sources

This change refactors HTTP repository source implementations. The
following changes have been made.

- CacheControl cache now lives within Authenticator.
- Authenticator manages unique sessions for individual netloc.
- CacheControl usage now respects disable cache parameter in repos.
- Certificate and authentication logic is now managed solely within
  Authenticator for source repositories taking advantage of recent
  enhancements.

These changes should allow for better handling of cases like those
described in #3041. Additionally, this forms the foundation for
unifying HTTP specific logic within the code base and possibly allowing
for migration of requests etc. if/when required.