Make KSampler preview images async to progress

### Feature Idea

As title described, sending KSampler live preview images over slow server-client connection may cause the progress displayed on client lag behind the actual progress displayed on the server CLI. This would make image generation seem slower than actual speed on the client side (i.e. client progress bar on KSampler node shows generation is at 50% progress while server is actually at 100% progress, then client needs to wait 2x the actual generation time before receiving the final output). By turning off live preview this behaviour of client lagging behind server disappeared, and this is not a problem for sufficiently fast server-client connections (i.e. loopback), so it must be that sending the live preview is blocking the network traffic on slow connections.

### Existing Solutions

_No response_

### Other

_No response_