[Feature] Multi-threaded PoW Hashing

[thoreaufyi]

• Checks if the user's PC supports the Worker API
• Checks how many threads the user's PC has
• Spawns a Worker for each Thread, and a unique 1 million nonce range
• When a worker finds the target PoW, all workers are terminated

It now finds PoW hashes at a comparable rate to the Gossip client

[hzrd149]

awesome, works perfectly

also I updated the PR to merge into the next branch since that is where the latest changes are