MINOR: server: define reverse-connect server

MINOR: server: define reverse-connect server

Implement reverse-connect server. This server type cannot instantiate
its own connection on transfer. Instead, it can only reuse connection
from its idle pool. These connections will be populated using the future
'tcp-request session attach-srv' rule.

A reverse-connect has no address. Instead, it uses a new custom server
notation with '@' character prefix. For the moment, only '@reverse' is
defined. An extra check is implemented to ensure server is used in a
HTTP proxy.