Revert "Products.CMFActivity.Activity.SQLBase: Drop "FOR UPDATE"."

This reverts commit 89aa2a6b.
The assumption that FOR UPDATE was unnecessary is wrong: when SQLDict
reserves similar activities, it will race against these primo-reservations.
So this change actually opens the possibility for duplicate activity
reservation.
Revert this change until (hopefully) a better fix is implemented.
43 jobs for feat/erp5pt in 0 seconds
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