Threads are the recommended way to scale CPU performance since HAProxy
1.8.

Official documentation says: « While "nbproc" historically used to be the only
way to use multiple processors, it also involved a number of shortcomings
related to the lack of synchronization between processes (health-checks, peers,
stick-tables, stats, ...) which do not affect threads. As such, any modern
configuration is strongly encouraged to migrate away from "nbproc" to
"nbthread". ».

Change-Id: I6f2e9d74e68703c8e0827e495945a75f020e1561