mod_proxy_balancer: follow up tp r1822509.

mod_proxy_balancer: follow up tp r1822509.

Rework server_rec ID so that it doesn't change on restart (or stop/start)
unless it's Host(s)/IP(s):port(s), ServerName and/or ServerAlias(es) changed.

The goal being to reuse SHMs (and persisted files) names as much as possible,
with minimal bindings to configuration changes (as far as mod_proxy_balancer
is concerned). So if the ServerName and first Host/IP:port are unique we use
that first, otherwise the ServerAlias(es) and other Host(s)/IP(s):port(s) are
also taken into account, and finally if that's still not enough the server
index is also used (pathological case handled for correctness with regard to
the underlying mod_slotmem_shm's reuse code).

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1823564 13f79535-47bb-0310-9956-ffa450edef68