sheep: get_request() for deferred response

With the new function, response message to clients can be deferred
safely without blocking the main thread.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
sheep, dog: prevent COW during snapshot creation and vdi deletion

Current generational reference counting scheme for garbage collection
of snapshot object requires mutual exclusion between COW operation and
snapshot creation or vdi deletion. This patch adds operations for
preventing/allowing COW temporally.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
dog: add a new subcommand "dog vdi object dump-inode" for diagnosing inode object

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>
update CHANGELOG.md for new object reclaim scheme and "dog vdi object dump-inode"

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>