Parallel pool import

This commit allow spa_load() to drop the spa_namespace_lock so
that imports can happen concurrently. Prior to dropping the
spa_namespace_lock, the import logic will set the spa_load_thread
value to track the thread which is doing the import.

Consumers of spa_lookup() retain the same behavior by blocking
when either a thread is holding the spa_namespace_lock or the
spa_load_thread value is set. This will ensure that critical
concurrent operations cannot take place while a pool is being
imported.

The zpool command is also enhanced to provide multi-threaded support
when invoking zpool import -a.

Lastly, zinject provides a mechanism to insert artificial delays
when importing a pool and new zfs tests are added to verify parallel
import functionality.

Contributions-by: Don Brady <don.brady@klarasystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: George Wilson <gwilson@delphix.com>
Closes #16093

include/libzutil.h

@@ -20,7 +20,7 @@
  */
 /*
  * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
- * Copyright (c) 2018 by Delphix. All rights reserved.
+ * Copyright (c) 2018, 2024 by Delphix. All rights reserved.
  */
 
 #ifndef	_LIBZUTIL_H
@@ -79,6 +79,8 @@ typedef struct importargs {
 	boolean_t can_be_active; /* can the pool be active?		*/
 	boolean_t scan;		/* prefer scanning to libblkid cache    */
 	nvlist_t *policy;	/* load policy (max txg, rewind, etc.)	*/
+	boolean_t do_destroyed;
+	boolean_t do_all;
 } importargs_t;
 
 typedef struct libpc_handle {