srm: Clean up SRMServerV2

- Make fields private and final
- Use class name as name of the logger and make logger static and final
- Catch specific excptions
- Improve error messages, both as logged and returned to the client
- Use varargs when possible
- Treat srmPing differently than other requests

srmPing is special as it cannot return a failure. Therefore the failure
code had a special case to return a regular ping reply even in case of
failure (eg authentication failure). Rather than trying to force srmPing
into the regular handler processing, this patch adds an implementation
of srmPing directly in SRMServerV2 and completely bypasses the usual
request processing. Besides speeding up srmPing processing, this allows
us to remove the workaround in the failure path.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6026/