OPENDJ-879 Add HTTP access log


Warn about non existing HTTP access logger when the HTTP connection handler is starting (was when loggers are initialized).


LoggerConfigManager.java, HTTPConnectionHandler.java:
Moved the warning about non existing HTTP access logger from LoggerConfigManager.initializeLoggerConfig() to HTTPConnectionHandler.startServer().

HTTPAccessLogger.java:
Made getHTTPAccessLogPublishers() public.