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>.