Add soft and hard TTL to schemarouter

The schemarouter shard cache only has a hard TTL value for refreshing the cache entry. Having both a soft TTL and a hard TTL would allow the entry to be refreshed without blocking other users if an old entry is available. This would also allow the cache to discard old entries completely that have exceeded the hard TTL value. This would free up the shard cache entries for users that are not in constant use.