Stale share detection flags non-stale shares

[luke-jr]

Probably it is flagging all shares based on work before a longpoll, which are not necessarily stale.

I suggest 2 different "levels" of "staleness":

1. the pool would rather we not work on this anymore
2. the pool will reject results from this as stale

Obviously level 2 is pool-dependent, so perhaps some new flag on longpoll responses is needed to determine "are shares on old works now invalid" ("submitold" can and is used to try to keep stale rate graphs accurate, but shouldn't inhibit level 2 stales)

[luke-jr]

OK, this looks uglier than I expected. There are already the 2 levels of staleness I proposed, but the logic for them is just totally wrong. Probably need to rewrite the stale_work function.

[luke-jr]

Fixed in b69181f