[FEATURE] Introduce volume live upgrade including engine

[innobead]

          The issue mentioned in https://github.com/longhorn/longhorn/issues/3125 is due to that `fsfreeze` freezes the mount point of the given path. Despite the [man page](https://linux.die.net/man/8/fsfreeze) stating the path is the pathname of the mount point, it can be any path. 

The potential race condition can be addressed by adding a device mapper (dm) device on top of the iSCSI block device. As a result, the block device was exported as a Longhorn volume.

If one wants to take a snapshot of the volume, it can execute dmsetup suspend and the filesystem is automatically frozen (code). In addition to preventing the race condition, the DM device can be used to live migrate a v1 volume to another instance-manager pod as described in #6001 (comment). Its disadvantage is that it cannot be applied to existing volumes.

Any feedback is appreciated. Thank you.

cc @shuo-wu @innobead @james-munson @WebberHuang1118 @Vicente-Cheng

Originally posted by @derekbit in #2187 (comment)