5269 zfs: zpool import slow
While importing a pool all objsets are enumerated twice, once to check
the zil log chains and once to claim them. On pools with many datasets
this process might take a substantial amount of time.
Speed up the process by parallelizing it utilizing a taskq. The number
of parallel tasks is limited to 4 times the number of leaf vdevs.

*** 58,67 ****
--- 58,68 ----
  extern zio_t *vdev_probe(vdev_t *vd, zio_t *pio);
  
  extern boolean_t vdev_is_bootable(vdev_t *vd);
  extern vdev_t *vdev_lookup_top(spa_t *spa, uint64_t vdev);
  extern vdev_t *vdev_lookup_by_guid(vdev_t *vd, uint64_t guid);
+ extern int vdev_count_leaves(spa_t *spa);
  extern void vdev_dtl_dirty(vdev_t *vd, vdev_dtl_type_t d,
      uint64_t txg, uint64_t size);
  extern boolean_t vdev_dtl_contains(vdev_t *vd, vdev_dtl_type_t d,
      uint64_t txg, uint64_t size);
  extern boolean_t vdev_dtl_empty(vdev_t *vd, vdev_dtl_type_t d);