Add slow disk diagnosis to ZED

Slow disk response times can be indicative of a failing drive. ZFS
currently tracks slow I/Os (slower than zio_slow_io_ms) and generates
events (ereport.fs.zfs.delay).  However, no action is taken by ZED,
like is done for checksum or I/O errors.  This change adds slow disk
diagnosis to ZED which is opt-in using new VDEV properties:
  VDEV_PROP_SLOW_IO_N
  VDEV_PROP_SLOW_IO_T

If multiple VDEVs in a pool are undergoing slow I/Os, then it skips
the zpool_vdev_degrade().

Sponsored-By: OpenDrives Inc.
Sponsored-By: Klara Inc.
Reviewed-by: Tony Hutter <[email protected]>
Reviewed-by: Allan Jude <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Co-authored-by: Rob Wing <[email protected]>
Signed-off-by: Don Brady <[email protected]>
Closes #15469 

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