Add Keepalives to Contour's gRPC serving

[youngnick]

In addition to #1744, adding TCP keepalives to Envoy, we need to add some form of keepalive to the Contour -> Envoy communication, to detect dead connections and prune them.

Initial investigation:

It doesn't really seem a good idea to mess with the TCP keepalive settings on the server side, as that's a property of the connection, not of the server (it seems).

Looks like we can add gRPC keepalives pretty easily since we already have a way to add extra server options to the gRPC listener. There's two parameters we really need to set, here are my suggestions about default values:

• Time (how long to wait before attempting a ping): 1 minute (This is an idle timer)
• Timeout (how long to wait before considering the connection dead: 20s (default, seems reasonable).

[youngnick]

Closed by #1758