This is better than using `get().is_some()` since we skip
decoding the storage value

This should be added back at some later point in time, but right now
the code doesn't allow for this behaviour.

Merge branch 'hc-start-sub-pallet' of github.com:paritytech/parity-bridges-common into hc-start-sub-pallet

We now require a justification for headers which _enact_ changes
instead of those which _schedule_ changes. A few changes had to
be made to accomodate this, such as changing when we check for
scheduled change logs in incoming headers.

Prevents a panic where if the header being imported and `best_finalized`
were the same header the ancestry checker would return an empty list. We
had made an assumption that the list would always be populated, and if this
didn't hold we would end up panicking.

If an ancestor which was newer than the child header we were checking we
would walk all the way to genesis before realizing that we weren't related.
This commit fixes that.

This trait was a remenant of the original design, and it is not required
at the moment. Something like it should be added back in the future to
ensure that other chains which conform to this interface can be used
by higher-level bridge applications.