Multi-threaded wallet scan

[fireice-uk]

Upgrade multithreading in the wallet to something that has actually a fighting chance of maxing out the CPU.

• Use of 64 bit lib is conservative, there is no way to produce anything but false negatives if there is an exploitable problem there.
• We strip away the abstraction layers in 64 bit code (mostly to avoid giving a blueprint for a parallel hw wallet implementation to guys that should really have been able to figure it out on their own)
• Bottleneck now is the block feed speed
• Time improvement will vary depending on the point above. HDDs won't see a lot. High IOPS NVMe SSDs will improve most. Overall 2 to 5x is a good ballpark figure so far.