Provide a way to process signals in order with multiple channels

[longquanzheng]

Right now there is no way to do that. Even thought server has recorded and delievered signal in order.
There seems to be some limitation in the current programming model with Selector.

I have seen this being needed several times. The current workaround is to let user merge multiple channels into a single one, which is not convenient to use.

See some context https://uber-cadence.slack.com/archives/CM4K75Y7R/p1623163349017500