Startup ordering between proxy and slotmem

[Rainer Jung]

I think slotmem is only a dependency for mod_proxy_balancer, not 
mod_proxy itself, so it seems safe (and more correct) to move the 
aszPred entry from mod_proxy.c to mod_proxy_balancer.c:

http://people.apache.org/~rjung/patches/mod_proxy-slotmem-order.patch
OK?