client: fix race with invalidations when starting a new transaction on ZODB 5
This requires ZODB >= 5.6.0
@jm, we will need this commit for wendelin.core 2 to be used with ZODB4 as well. With ZODB4 for
zconn_atfunction, we will have to manually do things that are done in upstream ZODB#307, and the property that all invalidations till
headare guaranteed to be already processed and completed after
head = zstor.lastTransaction()
is needed for the implementation (the same way as it is required for
So please cherry-pick this patch into v1.12 branches (
old-proto, etc) that are in production use currently.
Backported to old-proto branch.