Check conditions in AuthSlave and UserDBSlave
As stated in issue [#432 (closed)|https://jira.ow2.org/browse/#432], it is interesting to secure the Slave authentication. In our setup, the portal is behind a load-balancing proxy which also serves as a ssl endpoint. We would like to limit the Slave backend only for requests through this proxy. In other words only for requests which have X-Forwarded-By header with correct IP. This IP is not the remote address.
I propose a new pair of setting which allows to set the name a and content of a custom header in the same manor as the checkIP function does with the client address. Patch is attached.