Move Filter::ReadContext to ClusterMap

[XAMPPRocky]

Converting from ClusterMap to Vec<Endpoint> for every packet uses up a good portion of our time on the CPU, we should remove it and instead provide it a read only view into the cluster map for filters.

quilkin/src/cli/proxy.rs

Line 368 in 56c54f3

let endpoints: Vec<_> = config.clusters.read().endpoints().collect();