initial dynamic config not checked/restored if lost

[minrk]

Bug description

If the initial dynamic config set by setup_dynamic_config is lost, TraefikProxy never checks and restores the config. It has all the information needed to do so, it just doesn't check.

This can happen e.g. when the storage backend restarts without persistence (seen in #242) or there is data loss.

Restarting the Hub works to restore everything.

How to reproduce

1. Launch redis-server with no persistence
2. launch jupyterhub with traefik_redis
3. restart redis-server (state lost)
4. attempt a launch, it won't work

Expected behaviour

dynamic config is checked periodically and recreated, just like the routing table. This should perhaps be added to check_routes

Actual behaviour

dynamic config can only be recreated by restarting the Hub