mgr/DaemonServer: throttle pgp_num changes based on misplaced %

mgr/DaemonServer: throttle pgp_num changes based on misplaced %

- block pgp_num increase if pg_num hasn't increased yet
- make no changes if there are inactive or unknown pgs
- make no changes if there are degraded pgs either.  this might be a bit
  conservative...
- calculate the magnitude of our adjusted based on the max_misplaced
  target.  this assumes a uniform distribution of objects across pgs,
  so not perfectly accurate, but hopefully close enough.

Signed-off-by: Sage Weil <sage@redhat.com>