Remove filteredServers var in favor of memoized Function

[CannonLock]

Might just be a anti pattern in my world but seems like trouble to store a calculated value in state.

I suggest that we memoize filteredServers with dependencies on the downtime maps to remove the potential for a desync.

filteredServers = map[string]filterType{}

// A map of active and future downtimes set by server (Origin/Cache) admin, with the key being the ServerAd.Name
serverDowntimes = make(map[string][]server_structs.Downtime)
// A map of active and future downtimes set by topology, with the key being the ServerAd.Name
topologyDowntimes = make(map[string][]server_structs.Downtime)
// A map of active and future downtimes set by the federation admin, with the key being the ServerAd.Name
federationDowntimes = make(map[string][]server_structs.Downtime)