MFV 247176, 247178, 247315:

MFV 247176, 247178, 247315:
Import metaslab_sync() speedup from vendor (illumos).

Illumos ZFS issues:
  3552 condensing one space map burns 3 seconds of CPU in spa_sync() thread
  3564 spa_sync() spends 5-10% of its time in metaslab_sync() (when not
       condensing)
  3578 transferring the freed map to the defer map should be constant time
  3579 ztest trips assertion in metaslab_weight()

References:
  https://www.illumos.org/issues/3552
  https://www.illumos.org/issues/3564
  https://www.illumos.org/issues/3578
  https://www.illumos.org/issues/3579

MFC after:	2 weeks