Adapt some query counts to their optimal value, in order to measure the
effect of the following commits on queries.

In module test_performance, some query counts were actually not correct:
the initial flush() done by the context manager assertQueryCount() may
prefetch some data to the cache, and that prefetching is not accounted
for in the query count.  This is very true when assertQueryCount() is
preceded by a cache invalidation.  We have to move the invalidation
inside the context manager, so that the prefetching is now counted.