mod_ldap: retry on an LDAP timeout during authn.

trunk patch: http://svn.apache.org/r1510084

Submitted by: covener
Reviewed by: jim, humbedooh


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1523264 13f79535-47bb-0310-9956-ffa450edef68