client/finality-grandpa/src/until_imported: Refactor BlockGlobalMessage

`BlockGlobalMessage` owns an `inner` which contains (1) a count for the
amount of outstanding blocks to be waited on and (2) the message itself.

Given that both is already wrapped in an `Arc` there is no need to keep
track of the outstanding blocks, given that it simply corresponds to the
amount of strong reference counts on the `Arc` itself.

This commit removes the atomic counter within `inner` and piggy backs on
the `Arc` reference counter instead.