dog: let sheeps prevent inode update requests during snapshot creation

The recent new GC algorithm requires mutual exclusion between inode
update and and snapshot creation. This patch let sheeps prevent inode
update requests during snapshot creation with VDI locking information.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
sheep: cleanup generational reference counting code and document

This patch does a little refactoring of generational reference
counting. It also fixes trivial problems of pseudo code in
doc/object-reclaim.txt.

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