Add 3 memory handlers to replace the scoreboard area of mod-proxy.

Note the mod_scoreboard.c just uses the actual scoreboard "shared" area
for the workers.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/httpd-proxy-scoreboard@423444 13f79535-47bb-0310-9956-ffa450edef68