Thread ExpressionContext to SortStage so that it can access 'tempDir'

The ExpressionContext for a find command is currently held on the CanonicalQuery. In preparation for allowing SortStage to perform an external sort, we need to pass the tempDir and allowDiskUse settings for the query through to SortStage. This should be done by passing the find command's ExpressionContext through to SortStage's constructor.