Fixing DNS memory leaks.

Some of the changes only clean up at termination to make perftools
happt, but there were some "real" leaks as well.

This fixes all DNS leaks I could reproducem, including most likely
what's reported in #534. Closing #534.

I'm also adding a new btest subdir core/leaks with tests requiring
perftools support. These don't compare against base lines but abort
whenever perftools reports a leak (with stack information to track it
down). Right now, these are passing.

testing/btest/core/leaks/dns.bro

@@ -0,0 +1,48 @@
+# Needs perftools support.
+#
+# @TEST-REQUIRES: bro  --help 2>&1 | grep -q mem-leaks
+#
+# @TEST-EXEC: HEAP_CHECK_DUMP_DIRECTORY=. HEAPCHECK=local bro -m -r $TRACES/wikipedia.trace %INPUT
+
+# Add the state tracking information variable to the connection record
+
+event connection_established(c: connection)
+	{
+	when ( local addrs = lookup_hostname("localhost") )
+		{
+		print "1a", c$id$resp_h, addrs;
+		}
+	timeout 100secs
+		{
+		print "1b", c$id$resp_h;
+		}
+
+	when ( local addrs2 = lookup_hostname("qq.ww.ee.rrrrr") )
+		{
+		print "2a", c$id$resp_h, addrs2;
+		}
+	timeout 100secs
+		{
+		print "2b", c$id$resp_h;
+		}
+
+	when ( local a = lookup_addr(c$id$resp_h) )
+		{
+		print "3a", c$id$resp_h, a;
+		}
+	timeout 100secs
+		{
+		print "3b", c$id$resp_h;
+		}
+
+	when ( local a2 = lookup_addr(1.2.3.4) )
+		{
+		print "4a", c$id$resp_h, a2;
+		}
+	timeout 100secs
+		{
+		print "4b", c$id$resp_h;
+		}
+		
+	}
+