This PR fixes an issue which might result in data lost in freezer.

Whenever mutation happens in freezer, all data will be written into head data file
and it will be rotated with a new one in case the size of file reaches the threshold.

Theoretically, the rotated old data file should be fsync'd to prevent data loss.
In freezer.Sync function, we only fsync: (1) index file (2) meta file and (3) head
data file. So this PR forcibly fsync the head data file if mutation happens in the
boundary of data file.