The Meta.backend_C_r_S_unchanged() method was sadly a bit of a mess.

It seems to have originally been used as a method to be called
when the backend is not changing, with an extra assumption that if the
configured backend's hash doesn't match the one in state, surely the
hash should just be updated as an option might have been moved to
command line flags.

However, this function was used throughout this file as 'the method to
load the initialized (but not necessarily configured) backend',
regardless of whether or not it is the same (unchanged). This is in
addition to Meta.backendFromState(), which is used to load the same
thing except in the main codepath of 'init -backend=false'.

These changes separate the concerns of backend_C_r_S_unchanged() by

1) Fetching the saved backend (savedBackend())
2) Updating the hash value in the backend cache when appropriate (either
   by leaving it to the caller to do themselves or by calling
   updateSavedupdateSavedBackendHash())

This allows migration codepaths to *not* update the hash value until
after a migration has successfully taken place.