Dict: Invalidate iterators during Clear()

If we don't do this, a RobustDictIterator owned by someone else holds
dangling pointers to the Dictionary that is being cleared.

Closes #3523

testing/btest/core/dict-iteration-expire-3523.zeek

@@ -0,0 +1,14 @@
+# @TEST-EXEC: zcat <$TRACES/smb/smb_many_open_files_500.pcap.gz | zeek -b -Cr - %INPUT
+# @TEST-DOC:  Regression test #3523; no output check, just shouldn't crash
+
+redef table_expire_delay = 0.1sec;
+redef table_incremental_step = 1;
+redef table_expire_interval = 0.1sec;
+
+redef record connection += {
+	recent_tcp: set[string] &default=set() &read_expire=3min;
+};
+
+event new_packet(c: connection, pkt: pkt_hdr) {
+	add c$recent_tcp[cat(pkt$tcp)];
+}