Do not become a store_client for entries that are not backed by Store.

Do not become a store_client for entries that are not backed by Store.

If we ignore cache backing when becoming a store client, then
StoreEntry::storeClientType() is going to make us a DISK_CLIENT by default.
If there is no disk cache or it cannot be used for our entry, we will assert
in store_client constructor. Prevent those assertions by checking earlier in
StoreEntry::validToSend().