MINOR: http: implement the max-keep-alive-queue setting

MINOR: http: implement the max-keep-alive-queue setting

Finn Arne Gangstad suggested that we should have the ability to break
keep-alive when the target server has reached its maxconn and that a
number of connections are present in the queue. After some discussion
around his proposed patch, the following solution was suggested : have
a per-proxy setting to fix a limit to the number of queued connections
on a server after which we break keep-alive. This ensures that even in
high latency networks where keep-alive is beneficial, we try to find a
different server.

This patch is partially based on his original proposal and implements
this configurable threshold.