Handle non-int pool difficulties better.

[jcvernaleo]

Closes #140

[jolan]

I was trying to trigger this yesterday with intel graphics but it kept mining at diff 1 on yiimp.ccminer.org.

I remember seeing pools sending diffs < 1 awhile back so maybe we need something like:

if diff < 1
  diff = 1
else
  diff = math.Floor(diff)

I'm not clear on what diff 0 would actually do. Wouldn't it send basically everything? I guess it's easy enough to check.

[jcvernaleo]

We already don't allow diff <= 0 so yeah, I should set to 1 in the 0<d<1 case.

[jcvernaleo]

As for testing, I've had better luck see non-int diffs at dcr.maxminers.net

[jcvernaleo]

FYI I've been running with this pr for around 5 hours without an issue which is the longer than I've lasted without hitting the crash in #135 in a long time.

[jolan]

I was a bit concerned that rounding down the difficulty would cause rejects but I didn't see that in my testing with multiple pools. Since the pool backend stuff is a blackbox I will just assume that they have a minimum difficulty and accept everything above that threshold. I guess the difficulty adjustments are just suggestions?

Anyway, works fine in my testing with multiple pools.

OK