Randomize `user_channel_id` for inbound channels

Randomize `user_channel_id` for inbound channels

Previously, all inbound channels defaulted to a `user_channel_id` of 0,
which didn't allow for them being discerned on that basis. Here, we
simply randomize the identifier to fix this and enable the use of
`user_channel_id` as a true identifier for channels (assuming an equally
reasonable value is chosen for outbound channels and given upon
`create_channel()`).