gdss-brick >> gh17 Redesign disk storage:

- Implement write_metadata_* functions in brick_metadata_store
  modules.

Details:
- brick_hlog.hrl:
  * Move callback_ticket/0 type from brick_hlog_wal module.
- brick_metadata_store, brick_metadata_store_leveldb:
  * Implement the following functions:
    ** write_metadata/2
    ** write_metadata_group_commit/2
- brick_blob_store, brick_blob_store_hlog:
  * Remove Key parameter from write_value/2.
- brick_hlog_wal:
  * Rename register_group_commit/0 to request_group_commit/1.
  * Add Pid parameter to write_hunk_group_commit/2 and request_group_
    commit/1, so that brick_metadata_store module can explicitly give
    original caller's pid to brick_hlog_wal rather than implicitly
    giving its pid.