mds: remove anchor table

use backtrace instead of anchors to find/open remote inodes

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
mds: remove discover ino

Anchor table was the main user of MDCache::discover_ino(), it has
been removed. MDCache::discover_path() can replace discover_ino()
in remaining places. This patch removes discover ino related code.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>