zookeeper: look for the backend name in the node data

this is instead of looking for it in the node name, which is properly metadata.
this allows us to always generate ephemeral sequence ids for nerve node names
and still have readable information in haproxy
allow leader election by controlling backends returned

this is an alternate approach to airbnb/synapse#29 which is not tied to particular
service watchers. as long as the watcher ads a numeric ID field to each backend hash,
a leader can be elected.

we do not enforce leader election; should we fail hard and return no backends in such
cases?