MINOR: backend: Add srv_queue converter

MINOR: backend: Add srv_queue converter

The converter can be useful to look up a server queue from a dynamic value.

It takes an input value of type string, either a server name or
<backend>/<server> format and returns the number of queued sessions
on that server. Can be used in places where we want to look up
queued sessions from a dynamic name, like a cookie value (e.g.
req.cook(SRVID),srv_queue) and then make a decision to break
persistence or direct a request elsewhere.

Signed-off-by: Nenad Merdanovic <nmerdan@haproxy.com>