tests/functional: add test for object reclaim

This patch also removes existing test 086. 086 tests the clone
functionality and it depends on a max number of clones. But the object
reclaim scheme removes the maximum number. Hence the test becomes
obsolete. This patch adds a new test for the object reclaim as 086.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
doc: add documentation of object reclaim algorithm

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
tests/functional: add a new test of the object reclaim for cloned VDIs

This patch adds a new test of the object reclaim algorithm. The test
creates branch of VDI tree and checks the GC works well for the case.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
tests/functional: new test of object reclaim for a case of continuously snapshotted VDI

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
update CHANGELOG.md for new object reclaim scheme

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>