Allow turn on both FSImage parallelization and compression

The feature added HDFS-14617(in Improve FSImage load time by writing sub-sections to the FSImage index. by Stephen O'Donnell) makes loading FSImage very faster.

But this option cannot be activated when turn on dfs.image.compress=true.

In my opinion, larger clusters require both settings at the same time.

For Example, the cluster I'm using has approximately 6 million file system objects and FSImage is approximately 11GB with dfs.image.compress=true setting.

If turn off the dfs.image.compress option, it is expected to exceed 30GB, in which case it will take a long time to move FSImage from standby to active namenode using high network resource.

It was proved in this jira(HDFS-16147 by kinit) that loading FSImage parallel and FSImage compression can be turned on at the same time.  (And worked well on my environment also.)

I created this new Jira and PR because the discussion in HDFS-16147 ended in 2021, and I want it to be officially added in the next release, instead of patch available.

The actual code of the patch was written by kinit and I resolved empty sub-section problem(see below comment of HDFS-16147) and added test code.

If this is not a proper method, please let me know another way to contribute.

Thanks.