Allow MaxConnectionAge on Server

[pingles]

gRPC issues highlighted in #425 mention a preference to controlling flow of updates of the connection addresses via MAX_CONNECTION_AGE.

This should be added as a flag to the server, and passed when creating the gRPC server:

grpc.NewServer(grpc.KeepaliveParams(keepalive.ServerParameters{MaxConnectionAge: maxConnAge}))

The default value is infinity (https://github.com/grpc/grpc-go/blob/master/keepalive/keepalive.go#L60) but could be overridden to force clients to re-resolve and reconnect more regularly.

Should help #425 along with updating the gRPC Go version.

[stefansedich]

@pingles do we want to keep the existing default of infinity or should we default to something more sensible that suits our needs better?