SSU2: Token improvements and fixes part 1

- Set cache size based on connection limit
- Track average inbound cache eviction time
- Set inbound expiration based on cache time
- Reduce max inbound expiration
- Fix saving inbound token sent after relay response or hole punch
- Dont send or save tokens if we are symmetric natted
- Sort persisted tokens by expiration so they are expired in correct order on reload
- Periodically expire tokens from cache
- Add getters to Token class
- Add missing case IPV4_SNAT_IPV6_UNKNOWN to EnumSets