gdss-brick >> gh17 Redesign disk storage:

- Revise brick_metadata_store API and responsibility of the interface
  module. Now brick_metadata_store acts as the factory and registry of
  gen_servers of an implementation module.

Details:
- brick_metadata_store:
  * Add get_metadata_store/1 and implement gen_server callbacks.
- brick_metadata_store_leveldb:
  * Now each gen_server process owns a LevelDB port driver. It repair
    and open a LevelDB database at start up and close the database at
    termination.
- brick_data_sup:
  * Supervise the singleton of brick_metadata_store gen_server.
- brick_hlog_wal:
  * Remove name field from the state record. This is a singleton
    gen_server, so name would not be important.