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: mj

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

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