crimson/onode-staged-tree: add invalid state to tree_cursor_t

crimson/onode-staged-tree: add invalid state to tree_cursor_t

If a key-value pair is erased from the tree, the according tree_cursor_t
is untracked and becomes invalid.

From Cursor point-of-view, it becomes an end Cursor if the internal
cursor is invalidated.

From Value point-of-view, it becomes untracked if the value is erased.

Signed-off-by: Yingxin Cheng <yingxin.cheng@intel.com>