Server active connection list: avoid duplicates

[agrover]

Follow-on to #920, the connection list is a VecDeque. This is good because things put on the list are handled in order, but bad because a single connection can be put on the list multiple times. Either waiting should be some type of collection that doesn't add duplicates (but still returns things in order added?) or we should take care to not add a connection to it, if it's already on the list.

[larseggert]

@mxinden I think this is OBE with your recent changes?

[mxinden]

Yes. Good catch. Thanks.