Update the on-disk format for the root key so that it's wrapped with a unique
per-key/per-server key encryption key. This is a bit of security theatre for the
current implementation, but it uses `go-kms-wrapping` as the interface for
wrapping the key. This provides a shim for future support of external KMS such
as cloud provider APIs or Vault transit encryption.

* Removes the JSON serialization extension we had on the `RootKey` struct; this
  struct is now only used for key replication and not for disk serialization, so
  we don't need this helper.

* Creates a helper for generating cryptographically random slices of bytes that
  properly accounts for short reads from the source.

* No observable functional changes outside of the on-disk format, so there are
  no test updates.