Actually construct the `Authorship::set_uncles` inherent

[rphmeier]

#2941 creates logic for a set_uncles inherent to set some uncles. But we'll want to actually create a piece of logic that can call client.uncles() and ignore any duplicates (i.e. those which have already been included). We can model off the code for the FinalityTracker inherent, which makes use of the client as well.

[kianenigma]

@andresilva isn't this same or similar to #3293 ?

[andresilva]

No it's different. Currently the authoring logic for BABE (and Aura but not relevant there) doesn't collect and include any uncle data. Like @rphmeier said this needs to be done by calling client.uncles() and calling set_uncles with it when we are proposing a new block.