dcache: httpd service, add RequestLog to Jetty handler

If the Jetty RequestLogHandler is not given a request log implementation, it logs at the WARN level a message and then sets an internal Null implementation.

Given that the log message is annoying, we have provided a simple implementation of the log which delegates to the slf4j logger at the TRACE level.

Testing:

Before patch, with logger for domain set to WARN or higher, we see the jetty warning

15 Oct 2013 08:45:28 (httpd) [] !RequestLog

With patch, this disappears.  Logging of request/response, set at trace, looks like this:

15 Oct 2013 09:41:39 (httpd) [] request: [GET /webadmin/cellinfo]@794262320 org.eclipse.jetty.server.Request@2f577b30; response: HTTP/1.1 302
Date: Tue, 15 Oct 2013 14:41:39 GMT
Expires: Thu, 01 Jan 1970 00:00:00 GMT
Pragma: no-cache
Cache-Control: no-cache, no-store
Location: http://localhost:2288/webadmin/cellinfo?1

Target: master
Request: 2.6
Patch: http://rb.dcache.org/r/6115
Require-notes: no
Require-book: no
Bug: http://rt.dcache.org/Ticket/Display.html?id=8050
Acked-by: Gerd