Allow using externally created K2Pow to generate a POST proof

[poszu]

It should be possible to provide a k2 proof of work to generate the POST with in order to avoid recomputing it.

Requirements:

• generate_proof() should accept an optional k2pow parameter (an array per each nonce group?)
• if k2pow is provided for given nonce group - use it
• otherwise - compute it

[poszu]

Lowering priority as distributed k2 pow is not finalized yet.