TODO comments updated.
No longer using store front / store back naming.
Refactored out separate vbucket_state.go file.

Towards a shorter vbucket.go file.
Error msgs & comments.
TODO note on compatcion.
Unifed key-range, vbstate, CAS into VBMeta struct.

This refactorization into a new VBMeta struct (which is JSON'able) is
a stepping stone to eventually atomically saving and loading all the
key metadata of a vbucket, and also allowing that metadata to be
snapshotted and replicated down a TAP stream.