gdss-brick >> gh17 (redesign disk storage):

Continue working on LevelDB-backed metadata DB.
- Add server-side timestamp to 'delete' and 'delete_noexptime' do_mods
  so that the metadata DB, which has versioned metadata, can store
  delete markers.
- Move the registory for the metadata DBs from gmt_hlog_local to
  gmt_hlog so that gmt_hlog_common can handle full_writeback/0 at
  the boot time, where no brick/hlog_local servers are available.
- Remove unused functions gmt_hlog_common:unregister_local_brick/2
  and permanently_unregister_local_brick/2.

Details:
- brick_ets:
  * Investigated and documented the request handling path in
    brick_server and brick_ets. Will use this info for a major
    refactoring.
  * Add server-side timestamp to 'delete' and 'delete_noexptime' do_mods.
  * Remove some debug logs and add some info logs.
- brick_server:
  * Add server-side timestamp to 'delete' and 'delete_noexptime' do_mods.
- gmt_hlog/gmt_hlog_local:
  * Move the registory for the metadata BDs from gmt_hlog_local to
    gmt_hlog.
- gmt_hlog_common:
  * Remove unused functions gmt_hlog_common:unregister_local_brick/2
    and permanently_unregister_local_brick/2.
  * Update add_metadata_db_op/2:
    1) delete and delete_noexptime will create a delete marker in
       the metadata DB.
    2) unimplemented do_mods will throw an error.