MINOR: server: propagate lb changes through srv_lb_propagate()

MINOR: server: propagate lb changes through srv_lb_propagate()

Now that we have a generic srv_lb_propagate(s) function, let's
use it each time we explicitly wan't to set the status down as
well.

Indeed, it is tricky to try to handle "down" case explicitly,
instead we use srv_lb_propagate() which will call the proper
function that will handle the new server state.

This will allow some code cleanup and will prevent any logic
error.

This commit depends on:
- "MINOR: server: propagate server state change to lb through single function"