there are default values in the interchange code, but they are all specified in the executor code too, so these defautls will
never be used. remove them as misleading.

see similar changes to process worker pool, PR #2973, for more detailed justification

needs to change zmq sockets test because that assumes the arbitrary defaults are present.
which is no longer the case.
but if you want to initialize an interchange that requires you to specify all this stuff, and want some arbitrary values,
then make those arbitrary values yourself.

client address parameter is now supplied by the executor - it was not before, and so the default/hard-coded value
now lives in the executor, not the interchange

…l, not multiprocessing

any downstream packaging will need to be aware of the presence of interchange.py as a new command-line invocable script
and this might break some build instructions which do not configure installed scripts onto the path.

this PR replaces keyword arguments with argparse command line parameters. it does not attempt to make those
command line arguments differently-optional than the constructor of the Interchange class (for example, worker_ports and
worker_port_range are both mandatory, because they are both specified before this PR)

i'm somewhat uncomfortable with this seeming like an ad-hoc serialise/deserialise protocol for what was previously
effecting a dict of typed python objects... but it's what process worker pool does.

see issue #3373 for interchange specific issue

see issue #2343 for parsl general fork vs threads issue

see possibly issue #3378?

…s behaviour

…ehaviour to pass. Not tested against a real hanging interchange though...

…ckle