httpd: fix engine-based responses that don't implement getInfo

On starting system-test, I observed errors like:

06 Jul 2013 08:42:19 (System) [] context:httpdSetup: line 18: Command failed ((3) java.lang.NoSuchMethodException:
diskCacheV111.services.web.PoolInfoObserverEngineV2.getInfo(java.io.PrintWriter) from ac_set_alias_$_3_16; caused by:
diskCacheV111.services.web.PoolInfoObserverEngineV2.getInfo(java.io.PrintWriter))

This error is printed for all the classes declared through the 'alias'
command as having type 'class', except HttpPoolMgrEngineV3.  This
class is not logged because it's the only HttpResponseEngine to
implement the getInfo method.

The effect of these error messages is that the corresponding aliases
fail to work; i.e., URIs with the path starting /billing,
/flushManager, /pools, /api and /info no longer return the expected
content.

The problem seems to come from patch http://rb.dcache.org/r/5690/
(committed on master as 0039ddbe9de626aff2d06f8a1900689b59f5b26c).
This mostly focused on fixing logging; however, it also includes a few
bug-fixes.  One such bug-fix was for the AliasEntry class, used by the
httpd service.  This class does have a bug (the ResponseEngineHandler
instance is used when invoking the getInfo method, rather than the
HttpResponseEngine instance), however the above patch fails to fix it;
instead it breaks the Engine-based httpd services as described above.

Rather than simply reverting the broken part of the patch, this patch
removes the use reflection altogether.  Instead, the CellInfoProvider
interface is used to mark those HttpResponseEngine classes that
provide cell information.  To achieve this, the CellInfoProvider is
moved from the 'dcache' project to the 'cells' project.  A number of
classes have minor updates that correspond to this change.

Target: master
Requires-notes: no
Requires-book: no
Patch: http://rb.dcache.org/r/5714/
Acked-by: Gerd Behrmann