Channel routing: revert non-S routing to random, with new API to opt into routed

[mgravell]

mitigate #2955

Reason for revert: many environments use very few channels, which can lead to a massively skewed subscriber distribution in cluster scenarios, rather than making good use of the horizontal broadcast feature

• by default: use round-robin (not channel-routing) for "non-sharded" pub/sub
• add new API for channel-routed literals/wildcards
• when publishing, if we're also subscribed: use that connection
• randomize where the round-robin starts, to better randomize startup behaviour