* Perfile LevelDB instance usage are changed to use single instance
per FLU server.
* machi_csum_file reference is managed with machi_flu_filename_mgr
as an aim to manage filenames with leveldb
* Not only chunk checksums, but the list of trimmed files are also
stored in LevelDB.
* Remove 1024 bytes file header; instead put any metadata into
LevelDB if needed.
* LevelDB `db_ref()` lifecycle is same as that of `machi_metadata_mgr`
* `machi_file_proxy` just uses it as it's passed at process startup
* There are several optimization space still left as it is
WIP
