net: run close callbacks in correct eloop phase

Instead of running the close callbacks seemingly synchronously instead
of when the handle has actually been closed by libuv, instead run the
callbacks in the uv__run_closing_handles() phase of the eloop.
cluster: remove custom close callback

Now that the net close callbacks don't run until after libuv has had a
chance to properly close the uv_handle_t the custom close callback in
cluster is no longer necessary.
dgram: emit close when handle is closed
child_process: emit close in proper phase of eloop
zlib: emit after properly closed