[Cassandra] Display tree check

[sylvlecl]

• Do you want to request a feature or report a bug?

Feature

• What is the current behavior?

We cannot print the whole filesystem tree structure efficiently and exhaustively (see for example orphan nodes which cannot be accessed by the API). That possibility would help maintenance.

• What is the expected behavior?

Provide a DISPLAY_TREE check in CassandraAppStorage which will output (in an "issue" description) the tree of nodes, for example in an ASCII formatted tree.
This can be done efficiently by getting all the content of the children_by_name_and_class table, which should be fairly small.

For each node, we should provide the ID, the NAME, and the PSEUDO_CLASS.

Note that we should be more tolerant than the normal reads with invalid values (null values ...) : in particular, we should be able to print orphan nodes and their sub-trees?

• What is the motivation / use case for changing the behavior?

Helping understanding AFS issues in production systems.