hibari >> GH24 - Update for Erlang/OTP R16.
- Replace "R13..R15" in the required_otp_vsn with "R16".
- Replace crypto:md5/1 with crypto:hash/2.
- Replace -opaque types with -type.
Merge branch 'dev' into gbrick-gh17-redesign-disk-storage

Conflicts:
	src/gmt_hlog.erl
	src/gmt_hlog_common.erl
gdss-brick >> gh17 (redesign disk storage):

Start to implement LevelDB backed metadata DB.

NOTE: This commit requires additional apps under the lib directory.
One of these apps is not open-sourced yet.
See README-gdss-brick-gh17.md in the hibari/hibari repository for
more information.

- gmt_hlog_local module:
  * Add 'metadata_db' field to the state record.
  * Create/open and close the metadata DB upon init/1 and terminate/2.
  * Add get_metadata_db/1, which returns the reference to the DB.
- gmt_hlog module:
  * Add log_name2metadata_dir/1.
  * Remove an hard-coded binary for hunk_header_v1.
- gmt_hlog_common module:
  * Add a WIP (Work In Progress) section with the followings:
    ** Update write_back_to_stable_storage1/3 to write the metadata
       (store tuple) to LevelDB.
    ** Add add_metadata_db_op/2 and metadata_db_key/1 functions.
    ** Add a debugging aid tool_list_md/2 function.
- brick_ets module:
  * Add detailed definitions to extern_tuple/0 and do_mod/0 types.