zfs: merge OpenZFS master-436ab35a5
- speed up writing to ZFS pools without ZIL devices (aa755b3)
- speed up importing ZFS pools (2d8f72d, a0e0199, cf0977a)
...
MFC after: 2 weeks
Reviewed by: mjg (partial)
Tested by: pho
Differential Revision: https://reviews.freebsd.org/D28677