HPCC-9535 Rationalize Roxie topology info

Fix regression in previous commit (not subscribing to multicast channels).

Also fixes HPCC-9566 - failure to recover after catching exception in
multicast layer.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #4559 from richardkchapman/roxie-multicast-socket

HPCC-9535 Rationalize Roxie topology info

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>