default collection is not encoded in case of mixed-version cluster

See linked CBSE-14831 for additional details, but here are the reproduction steps:

1. setup two node cluster with KV service with 6.6 version
2. add one 7.1 node
3. rebalance
4. bootstrap libcouchbase using 7.1 node
5. libcouchbase will get socket configured for collections, because kv engine from 7.1 node will ack collection support
6. libcouchbase then receive configuration, and sees that the config does not have "collections" capability and disables collections support for whole lcb_INSTANCE
7. send GET command, and libcouchbase will not add zero prefix for default collection
8. kv engine tries to interpret first byte of the key as a collection index, and returns COLLECTION_NOT_FOUND
9. libcouchbase silently retries expecting that this is just cold collection, that will be accessible in a moment
10. application gets timeout error