sheep: introduce sparse objects

We don't need to care about a fragmentation when
 - the object is unlikely to be accessed sequentially, and
 - the object is read-only.

In that sense, we can make the objects sparse if they are not data
objects and writable ones.

This fixes the problem that sheepdog consumes many disk spaces for
deleted vdi objects if your filesystem supports FALLOC_FL_PUNCH_HOLE.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
sheepdog proto: recycle old child_vdi_id for storing btree_counter

Now the area isn't used and clients (QEMU and tgt) don't care about
it, we can recycle it for storing the btree_counter.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
sheep: update store format version to v5

This patchset introduces two important change for sheep store:
1. generational reference counting: adding ledger object and expand
   inode object for generation reference
2. changing a place of btree_counter in inode object

Incrementing SD_FORMAT_VERSION in either of them would be
confusing. So this independent patch updates the number.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
tests: show ledger objects in list

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 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>
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>