Refactor txg history kstat

Refactor txg history kstat

It was observed that even when the txg history is disabled by
setting `zfs_txg_history=0` the txg_sync thread still fetches
the vdev stats unnecessarily.

This patch refactors the code such that vdev_get_stats() is no
longer called when `zfs_txg_history=0`.  And it further reduces
the  differences between upstream and the ZoL txg_sync_thread()
function.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #5412