Auto refresh route cache

[ShiKaiWi]

Describe This Problem

Currently, the route information in the CeresDB instance is configured with a TTL for auto refreshing. And here are some problems of this mechanism:

• The route entry for a dropped won't expire in time;
• TTL is hard to be determined: a too short TTL will impose a large route traffic to CeresMeta, and a too long TTL will make invalid route entry exist for a long time;

Proposal

Replace the TTL mechanism with auto refreshing with Shard Version:

• Let CeresDB instance fetches the versions of its shards from CeresMeta;
• And fetch the tables belonging to the shard and updating the route entry in the cache;

Additional Context

No response

[ShiKaiWi]

The problem is that the route cache used for proxy is involved with all the tables of the whole cluster, so we can't cache routing information of all the tables.