Handing off SLRU fsyncs to the checkpointer

Hi,

In commit 3eb77eba we made it possible for any subsystem that wants a
file to be flushed as part of the next checkpoint to ask the
checkpointer to do that, as previously only md.c could do.

In the past, foreground CLOG flush stalls were a problem, but then
commit 33aaa139 cranked up the number of buffers, and then commit
5364b357 cranked it right up until the flushes mostly disappeared from
some benchmark workload but not so high that the resulting linear
searches through the buffer array destroyed the gains. I know there
is interest in moving that stuff into regular shared buffers, so it
can be found via the buffer mapping system (and improve as that
improves), written back by the background writer (and improve as that
improves), managed with a proper replacement algorithm (and improve as
that improves), etc etc. That sounds like a great idea to me, but
it's a big project.

In the meantime, one thing we could do is hand off the fsyncs, but I'm
not sure if it's still considered a real problem in the field given
the new parameters.

Anyway, I had a patch for that, that I used while testing commit
3eb77eba. While reading old threads about SLRU today I found that
several people had wished for a thing exactly like that, so I dusted
it off and rebased it.