Recursive deletion can lead to phantom read until the file cache expires

[jiacheliu3]

Is your feature request related to a problem? Please describe.
In Dora the file metadata is cached.

Imagine the below sequence:

1. We have file /a/b/c in Dora and by alluxio fs ls /a/b/c, the file metadata is cached in a Dora worker.
2. Then we recursively delete alluxio fs rm -R /a/. This will talk to a worker, and that worker will recursively remove /a/ from the UFS too.
3. Then we can still see alluxio fs ls /a/b/c from the cache, until that expires. At this stage, /a/ does not exist in UFS. But some of its children may still exist in cache.

Even if when we delete rm -R /a/, we clear all metadata about /a/* on that worker, that purging does NOT happen to all other workers. So if /a/b/c/ hashes to another worker (not the same one where we delete /a/ and all /a/* metadata), that /a/b/c cache may still exist.

Once that cache expires, or if we ls with a refresh option, ls will reflect the real state and complain the file does not exist. This is because if there is no cache, Dora worker will contact the UFS and realize the file does not exist.

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