hammer2 - Change from splay -> red-black tree

hammer2 - Change from splay -> red-black tree

* Even though splay trees are faster they are not lookup-safe under a
  shared lock.  In order to consolidate the chain locks with the cache
  coherency state we need to make lookups shared-safe.

* This will allow me to move chain locks from exclusive to shared/exclusive
  with only modest surgery.