Create wrapper API for apr_random;

use in mod_lbmethod_heartbeat and mod_serf to
- replace some needles use of apr_generate_random_bytes
- remove code duplication


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1171247 13f79535-47bb-0310-9956-ffa450edef68