Support FTP keepalives

[Castaglia]

Since mod_proxy could be used to talk to FTP/FTPS servers over WANs and other "hostile" networks, it should automatically some of the keepalive tricks mentioned in the KeepAlives howto.

[Castaglia]

One of the issues/tricks for this feature, I think, would be in automatically detecting whether the remote FTP server can handle commands on the control connection while also handling a data transfer; some FTP servers can, some cannot.

[Castaglia]

Another consideration is whether mod_proxy should even do this. Arguably it is the responsibility of the client to use such keepalives. We'd just need to ensure that mod_proxy properly proxies those commands to the backend server per normal.