Fix major performance issue with add_record O(N^2)

Before, 1-2k record took ~10s and more than that was just painful, 5k took
forever. This records things to keep a dict of nodes with a set of records so
that we can quickly "jump" to the point we're looking for without having to
search. 10k records now takes ~5s.

tests/test_octodns_provider_ns1.py

@@ -196,7 +196,8 @@ class TestNs1Provider(TestCase):
         provider = Ns1Provider('test', 'api-key')
 
         desired = Zone('unit.tests.', [])
-        desired.records.update(self.expected)
+        for r in self.expected:
+            desired.add_record(r)
 
         plan = provider.plan(desired)
         # everything except the root NS