First pass at a stubzones bugfix

[kdima]

This PR addresses the following issue.
Say you have 2 stubzones registered in etcd the following way:

/skydns/com/test/dns/stub/com/othertest/a/stubzone1/a    1.1.1.1
/skydns/com/test/dns/stub/com/othertest/a/stubzone1/b    1.1.1.2
/skydns/com/test/dns/stub/com/othertest/stubzone2/b       1.1.1.1
/skydns/com/test/dns/stub/com/othertest/stubzone2/a       1.1.1.2

Current implementation will consider these records duplicate and one of the stubzones will not be available.

To avoid this when we are looking up stubzone records we keep track of the subdomain in this case stubzone1.a.othertest.com and stubzone2.othertest.com and dedup records only inside the same subdomian.

I am not sure if this should be behind the flag stub or this can work as a general implementation.

[kdima]

This is the first pass if the implementation looks reasonable I will add same logics to etcd3 backend.