update luajit to luajit2

[lhy1024]

Signed-off-by: lhy1024 [email protected]

As this comment says, sysbench cannot run above 2000 threads with luajit, so upgrade it to luajit2

[Yui-Song]

@akopytov Could you please have a look at this? Can we merge it? As it's quite useful for large cluster testing. Many thanks!