Improve `ImportQueue`

[bkchr]

The import queue is for importing a block that comes over the network. This works relative good in Substrate, however with #445 we need to share this import queue. In the pr this is done by implementing a poor man `SharedImportQueue.

A much better solution would be to:

1. Improve the ImportQueue trait. Instead of having this poll function, every import request should return a Future that resolves to the results.
2. The ImportQueue should be cloneable by default.
4. Profit!