Avoid copying endpoints list for every packet

[iffyio]

Currently, we make a copy of the list of endpoints for every packet that flows through the system
https://github.com/googleforgames/quilkin/blob/master/src/proxy/server.rs#L155

this isn't ideal since the copies can be expensive and the list sizes can be significant.
We already wrap the list within a smart pointer like an Arc, and clone that instead of cloning the list and expose the wrapper to the rest of the code and filter chain - so that the overhead becomes a counter increment vs of a list copy.
e.g

pub struct Endpoints(Arc<Vec<EndPoint>>);

and pass this type in DownstreamContext as endpoints.

[markmandel]

Makes sense to me!