Perform all per-LDAP-backend related memory allocations in a standalone pool,

Perform all per-LDAP-backend related memory allocations in a standalone pool,
provide a local method to completely remove an LDAP backend connection so
we can someday manage/dispose of extra connections in a reasonable way.

Clarify some commentary around the existing murky close/cleanup API
methods.

Minor bump for new members appended to util_ldap_connection_t, which is not
allocated by consumers of the API.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@599164 13f79535-47bb-0310-9956-ffa450edef68