add threading support (webworkers) and make async versions of expensive operations

[jasonswearingen]

node v10 adds webworker support. all modern browsers now have it too.

create async versions of jsonx and make it the default usage.

[jasonswearingen]

as of node v10.15.x this is still highly experimental and not enabled by default: https://nodejs.org/docs/latest-v10.x/api/worker_threads.html

revisit once node v12 is released.

[jasonswearingen]

v18: PoC complete using the npm threads package. However might need to change. more work needed.