Before this patch 'zsync(storage)' was effectless for ZEO storages, it
didn't synchronize the client with the server. This patch fixes 'zsync',
so that it also performs synchronization of ZEO clients.

Background information:
=======================

2006 the sync mode of ZEO has been removed:

  nexedi/ZEO@629b0667

and only async mode was supported from then. This means, that the "sync"
method of ZEO.ClientStorage was in fact effectless. In ZEO 5 the
"server-sync" option has been added:

  https://github.com/zopefoundation/ZEO/pull/63

Setting this option to 'True' makes the 'sync' method performing a
"server round trip, thus causing client to wait for outstanding
invalidations" [1]. In this patch we imitate the effect of this flag
for both ZEO 4 and ZEO 5.

[1] https://github.com/zopefoundation/ZEO/blob/423cb8563be3e1ee0bb4297ee980d9b74f09c710/src/ZEO/ClientStorage.py#L225-L226

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/reviewed-by @kirr
/reviewed-on nexedi/wendelin.core!13