Introduce configuration groups to allow inheritance by virtual hosts of

Introduce configuration groups to allow inheritance by virtual hosts of
database configurations from the main server.  The post_config hook function
determines the minimal set of distinct configurations necessary so that
database connection pools are shared between virtual hosts whenever possible.

The SQL statements which should be prepared for each database connection
are now stored in a hash for each virtual host during the configuration
phase, and these hashes are merged in the normal manner using
apr_hash_overlay() with that of the main server.  This allows for statements
to be de-registered by DBDPrepareSQL, if desired.  The post_config hook
function then compares the statements registered for each virtual host
when determining if a separate configuration group is required.  The
changes in r424798, r432560, r432562, and r466641, which still have problems
with configuration inheritance, are therefore no longer necessary.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@503931 13f79535-47bb-0310-9956-ffa450edef68