Sequential nodes

By always creating zk nodes as sequential nodes, we get some advantages:

* there's no more race condition between session expiration and re-claiming of a node name (so we can revert a previous change)
* we can run multiple nerve processes per box, and multiple watchers per service, without node name clobbering ( @brndnmtthws )
* we can easily support leader/master elections (see airbnb/synapse#43)

This PR accomplishes the goals of #28 without adding a config option. It would close #28 

@Dareen @airbnb/sre