It is a legacy API from c*. Since we can wait for the
update_pending_ranges to complete, we can wait for it directly instead
of calling block_until_update_pending_ranges_finished to do so.
Also, change do_update_pending_ranges to be private.
Message-Id: <ac79b2879ec08fdcd3b2278ff68962cc71492f12.1458040608.git.asias@scylladb.com>