We write a lot of 4-bytes prefixes to file which call many system calls.
We should use a buffer and only write to file if the buffer is full or
finish writing. nsIBufferedOutputStream is a good candidate to do that
MozReview-Commit-ID: CzGOd7iXVTv
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extra : source : 8f2b4efc5f0d3191a7e80d9324933621e111b44a