Listener drops all connections when per filter config updated with RDS

*Title*: *Listener drops all connections when per filter config updated with RDS*

*Description*:
Having simple working setup:
```
static_resources {
listeners {
name: "listener"
address {
socket_address {
address: "0.0.0.0"
port_value: 8080
}
}
filter_chains {
filters {
name: "envoy.http_connection_manager"
typed_config {
[type.googleapis.com/envoy.extensions.filters.network.http_connection_manager.v3.HttpConnectionManager] {
stat_prefix: "ingress_http"
rds {
config_source {
ads {
}
initial_fetch_timeout {
seconds: 3
}
resource_api_version: V3
}
route_config_name: "xds-config-backend"
}
http_filters {
name: "dropbox.envoy.waf"
typed_config {
[type.googleapis.com/dropbox.envoy_dbx.WafConfig] {
}
}
}
http_filters {
name: "envoy.filters.http.router"
}
// ... less important... //
```
Everything works well, including updates sent by XDS to Envoy while there is no "per vhost filter config overrides" - envoy successfully and without any impact to traffic able to update route configuration.

However, if route configuration **contains** per virtual host overrides, like
```proto
resources: <
[type.googleapis.com/envoy.config.route.v3.RouteConfiguration]: <
name: "xds-config-backend"
virtual_hosts: <
name: "test1"
domains: "test1"
routes: <
match: <
prefix: "/test1"
>
route: <
cluster: "test_cluster"
>
>
typed_per_filter_config: <
key: "dropbox.envoy.waf"
value: <
[type.googleapis.com/dropbox.envoy_dbx.WafConfig]: <
>
>
>
>
```
Listener re-establishing all connections (`downstream_cx_active` gauge):

To me it seems like listener re-creates `HttpConnectionManager` and it somehow causes old connections to close.

Is it expected behavior?
Is there some parameter that we can adjust?
Any other ideas?

I'd be happy to provide more details, if needed.

Thanks