Create new name store for label names.

o Introduce "token" to describe property keys,
  relationship type names, and label names.
o Rename PropertyIndex to PropertyKey[Token]
o Rename NameData to Token
o New record type LabelTokenRecord in new store LabelTokenStore.
o Dynamic store containing the actual label names is still separate
  from other, similar name stores.
o Extended consistency checking to visit LabelTokenStore and its name store.
o Still need to add tests that exercise new consistency checking.
o In consistency checking, renamed relationship label to relationship type.