mod_proxy_balancer: Fix balancers and balancer members reuse on

mod_proxy_balancer: Fix balancers and balancer members reuse on
restart when new ones are added.  PR 58024.

Since slotmem_create() issues a strict check on the size of an existing
slot before reusing it, it won't reuse existing balancers and members when
new ones are added during restart (whereas growth margins would allow it).
Fix this by using slotmem_attach() first and if it succeeds do the checks
based on the returned previous number of existing entries.

[Reverted by r1698123]

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1696960 13f79535-47bb-0310-9956-ffa450edef68