added pow to text notes, reactions and metadata events. pow is
mined async in a worker so that the main process does not freeze.
noxy profile images, link and image previews are now now only
invoked if an event has some valid work proof. noxy can decide
if there is enough work and whether or not to serve data for a
certain event.
target difficulty can be implemented in a later step, this change
only check if there is any valid nonce tag with commitment target
greater than 0.
