gdss-brick >> gh17 Redesign disk storage:

Summary:
- Remove brick local HLog files and write-back process for these
  files. Local HLog files have been replaced by LevelDB based
  metadata DB.
- At start up, brick server now loads metadata records (store tuples)
  from metadata DB.
- Remove checkpoint process and the shadow table from brick server.
  (Note: test cases are not yet updated)

Details:

- brick_data_sup and gdss_brick.app.src:
  * Remove the following configuration parameters:
    ** brick_check_checkpoint_max_mb
    ** brick_chekk_checkpoint_throttle_bytes
  * Remove 'cp_throttle' brick_ticket process.

- brick_ets:
  * Remove the following fields from #state{} record:
    ** check_pid, check_lastseqnum
    ** checkpoint_timer, checkpoint_opts
    ** shadowtab, shadowtab_name, bypass_shadowtab
  * Remove a do_mod 'delete_all_table_items' from type.
  * Remove handle_call({checkpoint, ...}, ...).
  * Remove handle_cast({checkpoint_last_seqnum, ...}, ...).
  * Remove handle_info(check_checkpoint, ...).
  * Remove do_chackpoint/2 and related functions.
  * Remove wal_scan_all/2 and related funs for do_init_second_half.
  * Add load_metadata/1 and related funs for do_init_second_half.
  * Remove a bunch of shadow table relating functions such as
    get_many_shadow/7 and fold_shadow_into_ctab/2.
  * Remove function clauses for #state.shadow_tab =:= undefined
    from my_insert_ignore_logging3/3 etc.
  * Comment out old debug gen_server call 'qqq_debugging_only'.

- brick_server:
  * Remove checkpoint relating functions:
    ** checkpoint/2
    ** checkpoint/3
    ** checkpoint_bricks/1,
    ** checkpoint_bricks_and_wait/1
  * Remove handle_call({checkpoint, ...}, ...).
  * Remove a parameter Checkpoint_p from
    handle_cast({ch_repair_finished, ...}, ...)

- gmt_hlog_common:
  * Change WB_BATCH_SIZE from 200 to 500. (Note: wirte-back interval
    is 30 seconds.)
  * Remove write_back_exactly_to_logs/2 and related functions.

- gmt_hlog:
  * Add do_repair_metadata_db/2 and now do_get_or_open_metadata_db/2
    calls it before opening the DB.

- brick_ticket:
  * Replace == with =:=.