Fix dev-service import queue

[JoshOrndorff]

This PR corrects a mistake that I introduced in #260 . I thought that the only difference between the cumulus import queue and the manualseal import queue was that the cumulus one checked the inherents. I reasoned that therefore the cumulus one was strictly better.

However, I was wrong. The cumulus import queue does not mark any imported blocks as best. This leads to endless chain forking in a dev-service-based network.

This PR fixes it by constructing a manual seal import queue in the dev service only.

Longer term I may modify the manual seal import queue in Substrate so that it does check inehrents, and marking blocks as best is optional. Then cumulus could use the manual seal import queue. For now this fix should work.

Checklist

• ❌ Does it require a purge of the network?
• ❌ You bumped the runtime version if there are breaking changes in the runtime ?
• ❌ Does it require changes in documentation/tutorials ?