Needs page lock without owner

[hkimura]

Marked with "// TODO(Hideaki) PAGE LOCK HERE"

In MasstreeComposeContext::install_snapshot_pointers(), we must take a page lock because the page might be split at the moment. For that, we need a page lock without owner.

The plan is to assign either some node or core ID to a special "no-owner".
Waiters then either spin remotely or locally if they observe such ID.
Now that SGI already (wow) has 256 sockets machines, we shouldn't use node ID for this purpose.
I'm sure one node won't have 256 cores, so probably core-ID 255 can be saved for this.

array/hash/sequential don't need this. Only masstree package needs it.

[hkimura]

Fixed by #91