Explicitly use targets without global registration

[buddly27]

The pyblish.util.publish() function (and all helpers for specific CVEI stage) can take a list of targets but pyblish.util._convenience_iter() uses the global registration in the backend, which causes a bunch of issues:

• Targets previously registered get de-registered when process ends (as described in #345)

>>> pyblish.plugin.register_target("foo")
>>> print(pyblish.api.registered_targets())
["foo"]
>>> pyblish.util.publish(targets=["foo", "bar"])
>>> print(pyblish.api.registered_targets())
[]

• It is impossible to run 2 processes simultaneously with different targets

>>> t1 = threading.Thread(target=pyblish.util.publish, kwargs={"targets": ["foo"]})
>>> t2 = threading.Thread(target=pyblish.util.publish, kwargs={"targets": ["bar"]})
>>> t1.start() # Register "foo"
>>> t2.start() # Register "bar" while t1 is still running...

It would be nice to modify the logic so that targets explicitly passed to these functions don't get registered globally. What do you think?