worker: convert to a proper singleton

On many places we've been assuming that there's only a single worker,
but we still often didn't utilize the property well.  To get the pointer
we used various ways, all even untyped:
 - __worker global variable in lua
 - uv_default_loop()->data
 - kr_request::daemon_context

Now we instead simply define a global typed pointer the_worker.

Nitpick: also worker_{init,deinit}() are reordered to correspond
to the order of the fields, etc.