interval-tree: add augmentation fns to the tree

interval-tree: add augmentation fns to the tree

An interval tree uses red-black tree as its base data structure and
follows all the properties of a usual red-black tree. The additional
params are:
1. An interval such as [low, high] per node.
2. A max attribute per node. This attribute stores the maximum high
   value of any subtree rooted at this node.

At any point in time, an inorder traversal of an interval tree should
give the port ranges sorted by the low key in ascending order.

This commit modifies the IRB_AUGMENT macro and it's call sites to make
sure that on every insertion, the max attribute of the tree is properly
updated.

Ticket 6792
Bug 6414