Add a line to put the backup into place as the run directory.
Minor comment cleanup, rewrite the loops so there's only a single call to
the underlying function (I suspect the old code was a left-over from when
we had to be writing a node behind the current walk point, and that's no
longer the case).
Checkpoints (well, writing the root page), require passing a list of
current checkpoints down to the reconciliation of the root page, and
we use WT_BTREE.ckpt for that purpose.   Assert that is never happening
in more than a single thread.