rgw: resolve bugs and clean up garbage collection code

rgw: resolve bugs and clean up garbage collection code

Does a number of things to clean up rgw gc code:

  * adds additional logging to make future debugging easier.
  * resolves bug where the truncated flag was not always set correctly
    in gc_iterate_entries
  * resolves bug where marker in RGWGC::process was not advanced
  * resolves bug in which gc entries with a zero-length chain
    were not trimmed
  * resolves bug where same gc entry tag was added to list for
    deletion multiple times

Fixes: http://tracker.ceph.com/issues/38454
Signed-off-by: J. Eric Ivancich <ivancich@redhat.com>