Fix short-lived txg caused by autotrim

Current autotrim causes short-lived txg through:

1. calling txg_wait_synced() in metaslab_enable()
2. calling txg_wait_open() with should_quiesce = true

This patch addresses all the issues mentioned above.

A new cv, vdev_autotrim_kick_cv is added to kick autotrim activity.
It will be signaled once a txg is synced so that it does not change 
the original autotrim pace. Also because it is a cv, the wait is 
interruptible which speeds up the vdev_autotrim_stop_wait() call.

Finally, combining big zfs_txg_timeout, txg_wait_open() also causes
delay when exporting a pool.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: jxdking <lostking2008@hotmail.com>
Issue #8993
Closes #12194

module/zfs/spa.c

@@ -9449,6 +9449,9 @@ spa_sync(spa_t *spa, uint64_t txg)
 
 	spa_update_dspace(spa);
 
+	if (spa_get_autotrim(spa) == SPA_AUTOTRIM_ON)
+		vdev_autotrim_kick(spa);
+
 	/*
 	 * It had better be the case that we didn't dirty anything
 	 * since vdev_config_sync().