Do not unregister the callback in the zk watcher

Addresses incompatibility of per_callback threading in ZK, the way that
synapse calls unsubscribe within the callback, and ruby 2.X really not
liking joining on the current running thread.

Should fix #137 #99 and possibly #94

lib/synapse/service_watcher/zookeeper.rb

@@ -84,8 +84,7 @@ def watch
       return if @zk.nil?
       log.debug "synapse: setting watch at #{@discovery['path']}"
 
-      @watcher.unsubscribe unless @watcher.nil?
-      @watcher = @zk.register(@discovery['path'], &watcher_callback)
+      @watcher = @zk.register(@discovery['path'], &watcher_callback) unless @watcher
 
       # Verify that we actually set up the watcher.
       unless @zk.exists?(@discovery['path'], :watch => true)