BTreeMap: rewrite key serializer, first step towards specialized key serializer
BTreeMap: update other classes to match serialziers
BTreeMap: no null keys, first step
BTreeMap: add node.keysLen()
BTreeMap: more masking keys into nodes
BTreeMap: add node.cloneXX methods
BTreeMap: contain key serialization
BTreeMap: move node splitting into node methods
BTreeMap: remove more references to leaf.keys
BTreeMap: remove more references to leaf.keys
BTreeMap: cut null on leaf right
BTreeMap: cut null on dir right
BTreeMap: cut null on leaf left
BTreeMap: cut null on dir left
Merge branch 'btree-no-null-keys' into release-2.0
BTreeMap: remove comparator