[GSoC 2017 - BTRFS Write Supports] Week #4 #5

I spent last 2 weeks mainly for researching and forming an complete idea so that something will not be changed much in the future and some coding. These things are what I did in the previous 2 weeks:

This is a companion discussion topic for the original entry at https://www.haiku-os.org/blog/hyche/2017-06-14_%5Bgsoc_2017_-_btrfs_write_supports%5D_week_4_5/