keepalive appears to be unused

I just noticed in passing that the function isc_nm_tcpdns_keepalive() is defined in the netmgr, but is never called by named. I think the intent was to call it when the client sent an EDNS TCP KEEPALIVE option, so that named would switch to using a longer timeout.

The keepalive system test missed this because it only tests option processing and configuration, not timeout behavior. It would be nice to address that as well, though since it would necessarily be timing-dependent, it might be very difficult to make the test reliable.