NewGSIIndexer manage new set of indexes under namespace->keyspace, also called as, pool->bucket.

During cluster version < 7.0 (mixed mode), scope and keyspace are expected to be only _default as non-default collections cannot be created during mixed mode.

A global API that is independent of keyspace.

Query calls this function on the startup.

ErrorIndexEmpty is index not initialized.

ErrorIndexNotAvailable means client indexes list needs to be refreshed.