Allow writers to block while the cleaner makes a little progress.

Allow writers to block while the cleaner makes a little progress.

We record how much progress is required, and allow to wait
for that much progress to happen at which point a checkpoint
happens.

Also add 'free_segs' similar to 'free_blocks' (and counting blocks)
which counts the number of blocks in free segs, not including
any current segs.  This forced allocators to wait sooner and
may be more appropriate.

Signed-off-by: NeilBrown <neilb@suse.de>