Task #8888: Fix queue options in FromBus and ToBus, and force the use of topics when providing Services (to prevent bugs)

Task #8888: Added open() and close() to RPC class to hide implementation details of __enter__ and __exit__

Task #8888: Service is a frombus listener, so inherit from new class AbstractBusListener -> refactored out duplicate code

Task #8888: added abstract class AbstractBusListener to listen on a bus, and let the user implement one simple handle_message method

Task #8899: Allow broker options to be specified in FromBus/ToBus constructor (final final fix for tests without auto-reconnect, hopefully)

Task #8899: Credentials now have their own class, and a specific function to convert to str and to pg.connect options

Task #8899: Propagate username, password, port to pg.connect. Also, return dbcreds["port"] as an int

This patch works around the C++ side of MessageBus where XML encoding of special chars
is not done, or perhaps not done if libxml++ is not built.
It'd be better to fix the C++ side than to refrain from encoding on the python side in even
murkier situations, but MessageBus is deprecated and I've spent enough time trying to fix it.

Testing: All MessageBus tests pass on cbt009.

Task #8809: update commented-out filename in CMakeLists.txt for ObservationStartListener systemd service file