Revert r1831218, the API garantees slotmem_attach() is called in child_init().

r1831394 is the right follow to r1830800 to preserve "inherited" slotmems in
children processes.

While at it, comment on the expectations from mod_proxy_balancer w.r.t.
slotmem_attach() implementations, and eventually how we could improve the API
later (w/o backporting to 2.4.x).



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1831396 13f79535-47bb-0310-9956-ffa450edef68