lookup returned nil.
Add a TODO to note where a future point of logging should occur once a
logger is present and a few additional comments to explain the program
flow.
Assuming the following output from a consul agent:
```
==> Consul agent running!
Version: 'v0.7.3-43-gc5e140c-dev (c5e140c+CHANGES)'
Node ID: '40e4a748-2192-161a-0510-9bf59fe950b5'
Node name: 'myhost'
```
it is now possible to lookup nodes by their Node Name or Node ID, or a
prefix match of the Node ID, with the following caveats re: the prefix
match:
1) first eight digits of the Node ID are a required minimum (eight was
chosen as an arbitrary number)
2) the length of the Node ID must be an even number or no result will be
returned.
```
% dig @127.0.0.1 -p 8600 myhost.node.dc1.consul.
myhost.node.dc1.consul. 0 IN A 127.0.0.1
% dig @127.0.0.1 -p 8600 40e4a748-2192-161a-0510-9bf59fe950b5.node.dc1.consul.
40e4a748-2192-161a-0510-9bf59fe950b5.node.dc1.consul. 0 IN A 127.0.0.1
% dig @127.0.0.1 -p 8600 40e4a748.node.dc1.consul.
40e4a748.node.dc1.consul. 0 IN A 127.0.0.1
% dig @127.0.0.1 -p 8600 40e4a74821.node.dc1.consul.
40e4a74821.node.dc1.consul. 0 IN A 127.0.0.1
% dig @127.0.0.1 -p 8600 40e4a748-21.node.dc1.consul.
40e4a748-21.node.dc1.consul. 0 IN A 127.0.0.1
```
We always did an update before which caused excessive watch churn, even
with our new fine-grained queries. This does a diff any only updates the
node and service records if something actually changed.
We can't actually return a fine-grained index from these tables unless
support is added for tombstones. Otherwise, the index could slip backwards
as things are deleted.
