Currently in a lot of the places in the code, we check both WT_CONN_IN_MEMORY and WT_BTREE_IN_MEMORY. However, in some places, we only check WT_BTREE_IN_MEMORY. It is recipe for errors. We should always set the WT_BTREE_IN_MEMORY for all the btrees in the in-memory database. Then we only need to check WT_BTREE_IN_MEMORY not both in the code.