Avoid `poll`ing completed futures in the `background-processor`

Avoid `poll`ing completed futures in the `background-processor`

`poll`ing completed futures invokes undefined behavior in Rust
(panics, etc, obviously not memory corruption as its not unsafe).
Sadly, in our futures-based version of
`lightning-background-processor` we have one case where we can
`poll` a completed future - if the timer for the network graph
prune + persist completes without a network graph to prune +
persist we'll happily poll the same future over and over again,
likely panicing in user code.