Refactored asynchronous events.

Refactored asynchronous events.

To align njs with other JS engines, async events are removed from njs
core. The following functions were removed: njs_vm_add_event(),
njs_vm_del_event(), njs_vm_waiting(). Instead the host is expected
to manage async events by itself.

In addition, the posted events are renamed to jobs, to better align with
the ECMA specs. The following methods are removed: njs_vm_run().
Instead, the host is expected to call njs_vm_execute_pending_job() in a
loop to execute pending jobs. The following functions were added:
njs_vm_enqueue_job().