Make difficulty 128 bit

[aivve]

Based on Monero and Boolberry impl.

Not sure in near future we will need this but why not?

Requires testing, so far SYNCHRONIZED OK with full POW validation

[aivve]

It will look like this when the difficulty hits uint64_t numeric limit

"cumulative_difficulty": {
  "hi":1,
  "lo":9899
},
"difficulty":100000

[aivve]

Or in plain text cumulative_difficulty: 18446744073709561515 which is larger than 18446744073709551615

[aivve]

And yes, there's no migration tool, this requires syncing from scratch from nodes on prev. version.