state: separate refcounts from migrations

[fwereade]

Refcount values are (1) implied by the rest of a model description and
(2) pure implementation details necessitated by mgo/txn. They've been
dropped from description.Application; refcount sanity is now maintained
implicitly during import by the add*Ops-level funcs.

Refcount operations themselves have been extracted to refcounts_ns.go;
not all of them are used, but there's a forthcoming CL that will use
them for charm refcounts, thus helping us to safely delete
unreferenced charms in the near future.

(Review request: http://reviews.vapour.ws/r/5375/)

[fwereade]

folded into PR5939