Optimize the procedure of querying backend info when many backends are down

[djshow832]

Development Task

Currently, TiProxy queries backend info by querying both TiDBTopologyInfo and TTL from ETCD every few seconds. It's fine when there are few useless backends.

However, in an auto-scaling workload, there are more and more useless backends, who has TiDBTopologyInfo on ETCD but not TTL. It may make querying ETCD very slow or even fail.

So I propose to query all TTL first and query the info by the exact path.