Verify that exec nodes delete blocks

Currently BufferedBlockMgr is a per-query object that automatically cleans up any blocks at the end of the query. Most exec nodes will call Block::Delete() to let BufferedBlockMgr know that the block is finished. However, it's not clear if this is always done because blocks are automatically cleaned up at the end of the query regardless of their state.

This task is to add validation to check whether this invariant is true, and if not, make the appropriate changes so that Block::Delete() is consistently called.