Track the number of auctioned slots which are currently active

[shawntabrizi]

For gilts, we want to know the number of slots which were won by auctions in the current block.

Some kind of tracking vector needs to be used for each of the different lease periods.

[kianenigma]

This is getting more and more important because of #5872. Can you provide a bit more direction on what needs to be done? the kusama staking rate is off by quite a bit it seems, because of this.