There are currently 3 different ways to find the WT_BTREE handle in WiredTiger: S2BT(WT_SESSION_IMPL *session), WT_CURSOR_BTREE.btree, and WT_CURSOR.BTREE.dhandle.handle, and they're not kept in sync.

The WT_CURSOR_BTREE.dhandle field was added as part of donald.anderson's work in WT-4042, and should always be available.

We should remove the WT_CURSOR_BTREE.btree field, and change the Btree layer code to use S2BT(session) to acquire the Btree, requiring the standard pattern of passing the current WT_BTREE reference as part of the WT_SESSION_IMPL structure.