add sorted migration (not complete)

Signed-off-by: Akira Hayakawa <hayakawa@valinux.co.jp>
implement-ed sorted writeback (but buggy)

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
fix: sorted writeback

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
add static to compare_migrate_io()

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
Merge branch 'feature/sorted-writeback' into develop
test: change to ruby 2.1.1

KNOWNBUGS disappeared

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>