Moves sudoku generation off of the main thread. Allows for multiple
generation requests (up to the number of physical CPU cores) to be
served in parallel. Uses the "physical-cpu-count" node package to
determine the numer of physical CPUs of the host.
Also introduces a timeout which causes too-difficult (or impossible)
generation requests to fail if they take more than 5 seconds to
complete. In effect soft-capping the complexity of generated puzzles.
