The implementation of list_with_delimiter for the in-memory object
storage assumed that paths returned from the BTreeMap keys that sorted
greater than the prefix given to list_with_delimiter and for whom
prefix_matches returned true would also have parts after the prefix.
This didn't account for paths that started with the prefix but didn't
immediately have the delimiter after the prefix: that is,
prefix = 1/database_name
would match the in-memory paths:
1/database_name/0/rules.pb
1/database_name_and_another_thing/0/rules.pb
The first path here *would* return some parts_after_prefix, but the
second path would not and the previously existing code would panic for
the added path in the list_with_delimiter test case.
fbf18813ac