* maybe_gc/2 is triggered at machi_file_proxy, when chunk is deleted
and the file is larger than `max_file_size`
* A file is deleted if all chunks except 1024 bytes header are trimmed
* If a file is going to be deleted, file_proxy notifies metadata_mgr
to remember the filename persistently, whose filename is
`known_files_<FluName>`
* Such trimmed filenames are stored in a machi_plist file per flu
* machi_file_proxy could not be started if the filename is in the
manager's list. Consequently, any write, read and trim operations
cannot happen against deleted file.
* After the file was trimmed, any read request to the file returns
`{error, trimmed}`
* Disclaimer: no tests written yet and machi_plist does not support
any recovery from partial writes.
* Add some thoughts as comments for repairing trims.
* State diagram of every byte is as follows:
```
state\action| write/append | read_chunk | trim_chunk
------------+----------------+------------------+---------------
unwritten | -> written | fail (+repair) | -> trimmed
written | noop or repair | return content | -> trimmed
trimmed | fail | fail | noop
```
This is simply a change of read_chunk() protocol, where a response of
read_chunk() becomes list of written bytes along with checksum. All
related code including repair is changed as such. This is to pass all
tests and not actually supporting partial chunks.
