dog: add an option "-o" to vdi track for tracking object with oid

This patch adds a new option "-o" to vdi track. With this option,
users can track objects with their object ids. This would be more
friendly for users than existing "-i" option of vdi track. Because
output of "vdi check" is based on object ids. Users can recover
lost/corrupted objects manually based on this tracking information
when there are no majority objects.

Example of output:
$ dog vdi track test -o 0076680700000000
Tracking the object 0x76680700000000 (the inode vid 0x766807) with 5 nodes

obj 76680700000000 locations at epoch 1, copies = 3
---------------------------------------------------
127.0.0.1:7001
127.0.0.1:7000
127.0.0.1:7002

obj 76680700000000 locations at epoch 2, copies = 3
---------------------------------------------------
127.0.0.1:7001
127.0.0.1:7000
127.0.0.1:7003

obj 76680700000000 locations at epoch 3, copies = 3
---------------------------------------------------
127.0.0.1:7001
127.0.0.1:7004
127.0.0.1:7000

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>