Improve secondary storage framework

Improve secondary storage framework

- Let the secondary storage framework create backend instances on demand and
  track their failure state - the framework will stop calling a backend after a
  connection error/timeout. This makes it possible for a backend to create a
  connection in the constructor if wanted.
- Added API and implementation for redacting secrets in secondary storage URLs
  and attributes. Passwords in URLs are now redacted in debug logs but not for
  "ccache -p" and "ccache -k secondary_storage".
- Adapted existing storage backends to the new APIs. In particular, the Redis
  backend could be simplified.
- Moved the SecondaryStorage class to src/storage/secondary, analogous to
  PrimaryStorage which is in src/storage/primary.