Write buffer metadata into file ASAP

[tagomoris]

As shown in #1589, file buffer chunks possibly make broken/empty .meta files for its metadata.
I found that some points to be improved to prevent such cases:

• write metadata into files just after initializing buffer chunk object
  • before: .meta content is written at the time of #commit or enqueue! - it's too late
• write metadata msgpack binary, then truncate .meta file into the size of metadata
  • before: .meta was truncated into 0 at first, then msgpack binary was written

[tagomoris]

The behavior of msgpack unpacker for non-truncated files is proven here: https://gist.github.com/tagomoris/4b22651072aadc74b0ab9ecdfcf658a2

[a loooooooooooooooooooooooong msgpack binary]
[overwriting short binary]oong msgpack binary] # => unpacker unpacks shorter one only

[repeatedly]

I tested this patch with several days and no problem on my mac.
So merged and will release rc version for testing at other environment.