drop body of events

[meejah]

For NS and NEWCONSENSUS events, some listeners might not care about the body. In these cases, the body is pretty big (about 1.2MiB) since it's a microdescriptor for all ~8000 relays in the consensus.

It could be useful to have an option to add_event_listener to simply drop the body IF we can avoid building up and saving the body in the parser. Obviously, we can't avoid actually receiving the data (because Tor will always send it).