PoW threshold will be invalid with new upcoming v21 change

[Joohansson]

The new PoW needs to be changed from ffffffc000000000 to 8x that or fffffff800000000. Even better if it can be set via a parameter because send/change blocks will use 8x while open/receive blocks only require 1/8x or fffffe0000000000.

Current threshold:

RaiBlocksWebAssemblyPoW/functions.cpp

Line 23 in 7eb7cf5

uint64_t threshold = 0xffffffc000000000;

Reference: https://docs.nano.org/releases/network-upgrades/#increased-work-difficulty
Reference tool: https://tools.nanos.cc/?tool=difficulty&basediff=ffffffc000000000&multiplier=8

PR for webgl: numtel/nano-webgl-pow#1