Remove engine to client circular reference

[sorpaas]

In ethcore, the Engine trait is usually invoked by Client. However, in some cases, an Engine would also need to query additional state, and requires a reference back to Client. We have Engine::register_client and Error::RequiresClient. Because this is circular, it may be better if we can remove it.

Currently we only use this feature for Aura engine in two places:

• In on_close_block, where we need to calculate how many empty steps have passed.
• In EpochManager::zoom_to function, where we need to get the epoch transition for the parent block.

The first one can be easily fixed by using the new AncestryIterator, possibly combined with set_metadata.

The second one is a little bit more complicated -- we would need to add an extra field to BlockDetails, recording the current epoch transition directly, and then use the same method we allowed engine to fetch finalized info to fetch current epoch transition. The complication arises because this time we cannot avoid a DB migration. But it indeed has its benefits -- the epoch_transition_for function can now be always O(1).

[adria0]

Closing the issue due to its stale state