Previously, if a delegate's namespace was fetched and its rad/sigrefs
was behind the current state, then it would fail to perform the fetch.
Instead, when the ancestry path is checked the outcome can be used
during the update:
1. If the tip is equal, then the update can be marked as skipped.
2. If the tip is behind, then the update can be rejected, iff the
policy does not allow it.
3. If the tip is ahead, then the update is a fast-forward.
4. If the tip is diverged, then the update results in a failure, iff
the policy does not allow it.
The protocol also prunes any delegates's updates when the `rad/sigrefs`
is behind and returns an error if they are diverged.
For any non-delegates, it will simply prune them.
Signed-off-by: Fintan Halpenny <fintan.halpenny@gmail.com>
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