Prevent async channels from sending streams after the service finishes

[mxgrey]

Currently it is possible for a user to implement an async service that sends its async channel outside of the function scope so that the channel can outlive the function scope. Then the channel could continue to be used to transmit new stream signals after the service has terminated.

This violates bevy_impulse's contract that services cannot produce stream data after their final output has been delivered.

We should put a mechanism into the async channel that allows us to close it once we have received the final output for the service call that the channel was created for.