ERP5Type.patches.CookieCrumbler: Fix balancer cookie logic.

Ever since the introduction of X-Balancer-Current-Server in SlapOS
haproxy configuration in:
  commit c7f104fbf1f5948928f5545286afbb94fc39102b
  Author: Cédric Le Ninivin <cedric.leninivin@tiolive.com>
  Date:   Tue May 26 13:27:33 2015 +0200

      erp5_cluster: leave ERP5 set the haproxy cookie

the balancer will overwrite this cookie's value, so do not compare it
to our name: balancer may be calling us a name completely unrelated to
the value getCurrentNode returns.
So simplify this function's logic to only do what the docstring says:
decide when to set and when to unset the balancer cookie, and let the
balancer decide the value.
This resolves cases where Zope would set the cookie over and over on
every request when getCurrentNode and balancer disagree on its value.