Smarter auction discovery and monitoring

Currently auction discovery and monitoring operates as a loop which kicks in on every new block and simply enumerates all auctions from 1 to kicks. This has several drawbacks:

• Deleted auctions are being constantly queried over and over again, which will make the keeper work really slow as the number of historical auctions grows.
• Output from bidding models doesn't get interpreted instantly, but only with each block. This can delay our bids by several seconds.
• ...

The plan is to replace it with a smarter mechanism, which should remember which auctions are already gone (and thus will not resurrect) and not query them constantly. At the same time any output from bidding models should get interpreted instantly.

[EdNoepel]

This will be complicated for flop auctions, as 0.2.12 introduces the opportunity of resurrecting a flop auction which completed with no bids.