Previously this was always set for all requests. The parameter is only
documented as supported for certain queries, so this was probably wrong.
Mongo version 4.2 fails update queries that have this parameter set making
mongoengine unusable there. Fixes#2148.
Using QuerySet directly would fail if QuerySetNoCache was used. Sadly the
test suite does not have a case where QuerySet would appear here, so adding
a test for this special case is not trivial.