lib: create new function set_vid_for_btree()

Create new function set_vid_for_btree() to replace some codes in
sd_inode_set_vid(), so the structure of code will be better.

Signed-off-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
lib: change interface sd_inode_set_vid()

sd_inode_set_vid() only set one 'vid' at before, we change it
to support 'set range of vids'.

Signed-off-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
lib: add cache version of writer and reader

Add new functions to repleace default writer and reader so it can cache
entire ext-node when set_vid_for_btree() need to access ext-node.

Signed-off-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>