PR63305: fix graceful restart crashes in LDAP

The cache destruction was not protected by the lock used by other
cache callers.

Pull the static cleanup function into util_ldap.c so it's convenient to 
use the existing locking.

Submitted By: Martin Fúsek <mfusek newps.cz>
Commited By: covener



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1856735 13f79535-47bb-0310-9956-ffa450edef68