URI Segments for top-level nodes need to be unique within site

Description

URI Segments for top-level nodes currently need to be unique within an entire site.

Steps to reproduce:
1. Create a node with UriSegment test.
2. Create a child of this node that has UriSegment equal to test-2.
3. Try to modify the parent node to have UriSegment equal to test-2.

Expected:
It is possible to change the UriSegment.

Actual:
The UriSegment gets a suffix of 8 hexadecimal digits.