Pipe filesystem writes in `lightning-persister` through `BufWriter`

Pipe filesystem writes in `lightning-persister` through `BufWriter`

We generally make no effort to ensure all writes are buffered in
lower-level objects, so wrapping write calls in `BufWriter` may
substantially improve performance in some cases. This is especially
important now that we block the sample node exit until the
`NetworkGraph` has been written out, which includes many small-ish
writes.

With this change, shutdown of the sample node on a relatively
underpowered device went from 15-30 seconds of CPU time to a second
or two, plus IO sync time.