Previously, an assertion was triggered in debug builds upon any attempt
to insert or remove a Dictionary entry while any iteration of that
Dictionary is underway and also even in cases where Dictionary membership
was not actually modified (and thus invalidates a loop).
Now, it emits run-time warnings regardless of build-type and only when
insert/remove operations truly change the Dictionary membership. In the
context of a Zeek script causing an invalidation, the warning message
also now helps pinpoint the exact expression that causes it.
