New content: advanced guide for improving performance of HHVM deployments

[fredemmott]

• use repo-authorative mode
• enable PGO - and tune retranslateall requests/seconds
• tune the jit*Size variables (this section is definitely beyond my abilities to write)
• consider using jit serdes
• tune HHVM machine pool size - this may mean decreasing the number of servers, if the JIT spends all its' time warming up; what's a good initial value?
• tune number of request threads
• consider CLI server mode instead of standalone CLI mode
• server/proxygen-related settings: ServerBacklog, ThreadCount, ThreadJobLifoSwitchThreshold,ThreadDrop* settings
• bolt
• update jemalloc

[Wilfred]

FWIW there's some repo auth docs at https://docs.hhvm.com/hhvm/advanced-usage/repo-authoritative