Always use right mem cxt when creating ColumnarReadState
Always use right mem cxt when creating ColumnarReadState

All the callers except columnar_relation_copy_for_cluster were already
switching to right memory context when creating ColumnarReadState.

With this commit, we embed that logic into init_columnar_read_state
to avoid further such bugs.

That way, we start using the right memory context for
columnar_relation_copy_for_cluster too.

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