[MEDIUM] add support for global.maxconnrate to limit the per-process conn rate.

[MEDIUM] add support for global.maxconnrate to limit the per-process conn rate.

This one enforces a per-process connection rate limit, regardless of what
may be set per frontend. It can be a way to limit the CPU usage of a process
being severely attacked.

The side effect is that the global process connection rate is now measured
for each incoming connection, so it will be possible to report it.