tool: rename kill_zk_session as zk_control

This is prepared to add more methods

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
zk_control: refactor the code

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
zk_control: add remove method

With remove method, we can remove nasty 5s sleep in the _kill_sheep(), will
boost tests a lot with zk.

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
checkpath: fix block comment false alarm

+* blabla.
+* blabla.
+*/

Above in the patch file is valid block comments

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
farm: remove xattr code in sha1.c

refcount of sha1 file for deletion makes use of xattr, but currently we don't
support snapshot deletion yet.

This patch remove xattr code completely because:
- deletion snapshots can be exercised by
  1. export the snapshots you want to keep to another snapshot storage
  2. delete the old snapshot storage.

- Assume storage for storing snapshots to support xattr is very lame, e.g., we
  we can't rely on Amazon's S3 storage to support kind of xattr.

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
farm: remove unused code in sha1.c

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
tests/functional: remove unnecessary _wait_for_sheep_stop

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
tests/functional: move zk node remove in _wait_for_sheep_stop

Then both _kill_sheep and _kill_all_sheep will make use of it

Signed-off-by: Liu Yuan <namei.unix@gmail.com>