sheep: introduce generational reference counting for object reclaim

Generational reference counting is an algorithm to reclaim data
efficiently without race conditions on distributed system.  This
extends vdi objects structure to store generational reference counts,
and increments the counts when creating snapshots.

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

This introduces ledger objects, which keeps track of the number of
outstanding references of each generation.  Sheep decrements a
generational reference count with a gateway request SD_OP_DECREF_OBJ,
and reclaims objects when there is no generational reference.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
sheep: decrement generational reference count on vdi deletion

This removes old vdi deletion code, which reclaims objects only when
all relevant snapshots are deleted, and uses a generational reference
counting algorithm.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
sheep: decrement generational reference count on copy-on-write

This decrements a reference count of the old data object when
allocating a new data object on CoW.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
sheep: remove max children limit

We no longer need it.  This also reduces the vdi header size.

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

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

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

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>