Switch to the Envoy proxy_protocol filter instead of using use_proxy_proto.

[kflynn]

There are three things going on this commit, but all of them need to be in one commit for things to work.

1. use_proxy_proto is no longer a VHost thing.
   a. This is why you see all the deletion of stuff in V2VirtualHost about use_proxy_proto.
   b. This is also why you see use_proxy_proto vanishing from all the calls instantiating a V2VirtualHost.

2. use_proxy_proto is instead now a V2Listener thing.
   a. This is OK because, at present, use_proxy_proto is a system-wide configuration. Later, it needs to be part of the per-listener config.
   b. This is why you see use_proxy_proto getting added to V2Listener.

3. V2ListenerCollection gains a get method, because
   a. using __getitem__ really didn't make sense, it was a design choice back when I thought more needed to be in the V2ListenerCollection class
   b. we really need to pass use_proxy_proto in on creation, and __getitem__ relying on two inputs would be even more absurd than it used to be.

Fixes #2348.

Documentation doesn't need to change. Testing was with KAT and using curl --haproxy to test the TLS termination + PROXY protocol case.

Tasks That Must Be Done

• Did you update CHANGELOG.md?
• Testing was manual -- we're still sorting out how to add a CI test for this.
• I didn't need to update docs.