bucket: Add more reconciler design improvements

- Remove ArtifactUnavailable condition and use Reconciling condition to
  convey the same.
- Make Reconciling condition affect the ready condition.
- Introduce summarizeAndPatch() to calculate the final status conditions
  and patch them.
- Introduce reconcile() to iterate through the sub-reconcilers and
  execute them.

Signed-off-by: Sunny <[email protected]>

Author: darkowlzz