GH-1019: deprecate icmp_conn params for ICMP events

Previously, a single `icmp_conn` record was built per ICMP "connection"
and re-used for all events generated from it.  This may have been a
historical attempt at performance optimization, but:

  * By default, Zeek does not load any scripts that handle ICMP events.

  * The one script Zeek ships with that does handle ICMP events,
    "detect-traceroute", is already noted as being disabled due to
    potential performance problems of doing that kind of analysis.

  * Re-use of the original `icmp_conn` record tends to misreport
    TTL and length values since they come from original packet instead
    of the current one.

  * Even if we chose to still re-use `icmp_conn` records and just fill
    in a new TTL and length value each packet, a user script could have
    stored a reference to the record and not be expecting those values
    to be changed out from underneath them.

Now, a new `icmp_info` record is created/populated in all ICMP events
and should be used instead of `icmp_conn`.  It also removes the
orig_h/resp_h fields as those are redundant with what's already
available in the connection record.

scripts/policy/misc/detect-traceroute/main.zeek

@@ -95,7 +95,7 @@ event signature_match(state: signature_state, msg: string, data: string)
 		}
 	}
 
-event icmp_time_exceeded(c: connection, icmp: icmp_conn, code: count, context: icmp_context)
+event icmp_time_exceeded(c: connection, info: icmp_info, code: count, context: icmp_context)
 	{
 	SumStats::observe("traceroute.time_exceeded", [$str=cat(context$id$orig_h,"-",context$id$resp_h,"-",get_port_transport_proto(context$id$resp_p))], [$str=cat(c$id$orig_h)]);
 	}