Introduce new config option LDAPTimeout to set the timeout for operations

Introduce new config option LDAPTimeout to set the timeout for operations
like bind and search.

For operations other than search, the support of the timeout depends on the
LDAP SDK supporting LDAP_OPT_TIMEOUT (e.g. OpenLDAP >= 2.4.4). An exception is
the ldap_simple_bind() in uldap_connection_open(): Here we put some extra
effort into detecting a timeout in a portable way, to catch an idle connection
having been dropped by a firewall.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@898102 13f79535-47bb-0310-9956-ffa450edef68