accept a reasonably live consensus for guard selection

Clients with clocks skewed far enough in the future to never get a live consensus, but still have a reasonably live one, end up downloading descriptors and then getting stuck on guard selection. This is a rather bad user experience because bootstrap progress appears to get stuck at 80% or 85% even though something rather fundamental (time of day) is wrong.

It's not clear that a reasonably live consensus is dangerous to use for guard selection, so always accept a reasonably live consensus instead of a live one for guard selection.

Ticket #2878 (moved) covers the case of deferring descriptor downloads if the consensus isn't live, which would also improve the UX but might not be necessary if we implement the solution in this ticket.