server: Improve style of announce/filter logic
server: Improve style of announce/filter logic

- Filter before potentially creating a swarm that is not needed
- Unify getSwarm()/createSwarm() into a getOrCreateSwarm() function
jhiesey/bittorrent-tracker,feross/bittorrent-tracker,LinusU/bittorrent-tracker,jhiesey/bittorrent-tracker,feross/bittorrent-tracker