os: write file journal optimezation

os: write file journal optimezation

Currently, there is single write thread for file journal, so it would be bottleneck.
It is important to keep logic of the journal write thread simple. According to the
implementation of transaction encoding, it is almost impossible that the write
bufferlist would be align. So write journal would call rebuild_aligned almost every time.
Because of the memory fragmentation, the bufferlist crc and rebuild would be bottleneck.

My implementation would move the complex logic out of journal write thread.

Signed-off-by: Xinze Chi <xinze@xsky.com>
Reviewed-by: Haomai Wang <haomai@xsky.com>