stack/erp5: enable server-sync for ZEO

For ZEO4, this was implemented as a monkey patch in ERP5, but this is
now supported in ZEO5, but not enabled by default.

The setting is only avaiable through ZEO's own ZConfig, but not with
ZODB's <zeoclient> so we also adjust the config to import ZEO and use
ZEO's <clientstorage>.
4 jobs for zope4py2 in 0 seconds
Status Job ID Name Coverage
  External
failed ERP5.CodingStyleTest-Zope4Py2

01:36:02

failed ERP5.UnitTest-Zope4Py2

01:20:21

failed SlapOS.SoftwareReleases.IntegrationTest-Zope4Py2

00:30:15

failed SlapOS.SoftwareReleases.IntegrationTest-Zope4Py2

00:34:16