performance/md-cache: Fix caching for root inode

Summary:
- `is_mdc_key_satisfied()` is returning 0 when it has not checked any of the keys
- This causes the cache'd value for the root inode to always be invalid (`mdc_xattr_satisfied()` returns 0, which causes us to jump to `uncached').
- In this diff we add a new option called "strict-xattrs", when enabled winds getxattr calls for those keys not present in our cache.
- This allows "special" getxattr commands (quota cli commands for example) to work when md-cache is enabled.
- This is a port of D4135452

Test Plan:
- Test on devserver and see latency improvements for root inode.
- Prove tests

Signed-off-by: Shreyas Siravara <sshreyas>

Change-Id: I8ff75595e821d7a714224b3b3dded23f0a93560a
Reviewed-on: http://review.gluster.org/16060
Smoke: Gluster Build System <jenkins.org>
CentOS-regression: Gluster Build System <jenkins.org>
Reviewed-by: Kevin Vigor <kvigor>
Tested-by: Shreyas Siravara <sshreyas>
NetBSD-regression: NetBSD Build System <jenkins.org>