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 <hutter2@llnl.gov>
Reviewed-by: Allan Jude <allan@klarasystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Co-authored-by: Rob Wing <rob.wing@klarasystems.com>
Signed-off-by: Don Brady <don.brady@klarasystems.com>
Closes #15469

lib/libzfs/libzfs.abi

@@ -5626,7 +5626,9 @@
       <enumerator name='VDEV_PROP_IO_N' value='44'/>
       <enumerator name='VDEV_PROP_IO_T' value='45'/>
       <enumerator name='VDEV_PROP_RAIDZ_EXPANDING' value='46'/>
-      <enumerator name='VDEV_NUM_PROPS' value='47'/>
+      <enumerator name='VDEV_PROP_SLOW_IO_N' value='47'/>
+      <enumerator name='VDEV_PROP_SLOW_IO_T' value='48'/>
+      <enumerator name='VDEV_NUM_PROPS' value='49'/>
     </enum-decl>
     <typedef-decl name='vdev_prop_t' type-id='1573bec8' id='5aa5c90c'/>
     <class-decl name='zpool_load_policy' size-in-bits='256' is-struct='yes' visibility='default' id='2f65b36f'>