Speed up second write in snapshot mode

[hongyuntw]

#357 We can pre-check if the bio is first write or not, only the first write need make read clone.

In the origin version
1 second write -> 1 read clone + 1 write (disk)

And this commit can optimize some second writes speed (if the second write bio bitmap is in memory) we can perform
1 second write -> 1 write (disk)

[Swistusmen]

Thanks, I think we are going to review it after filling the goals for the next release (you know first, new features, than optimizations). So your PR with feature test will go first, than mine new features, than probably another with bug fix, and than we could start improving code. So I think I will return here between mid of apirl and mid of may