refs #5801 merge 5801 (improve threaded stress test framework) to main: now the loader is used for sufficiently large tables, correctness tests fill tables with zeroes while performance tests fill them with bytes based on compressibility, and both use similar code paths for key/val generation for better consistency and readability


git-svn-id: file:///svn/toku/tokudb@51638 c7de825b-a66e-492c-adef-691d508d4ae1