netbox: fix wait_connected ignorance

After this patch d2468dac it became possible to
wrap an existing connection into netbox API. A regular
netbox.connect function was refactored so as to reuse
connection establishment code.

But connection should be established in a worker
fiber, not in a caller's one. Otherwise it is
impossible to do not wait for connect result.

The patch just moves connection establishment into a
worker fiber, without any functional changes.

Closes #3856