A subscription from Slap.SlateDB.subscribe/3. Its :ref is the
reference in every message the subscription sends; match on it to tell
subscriptions apart:
{:ok, %Slap.SlateDB.Subscription{ref: ref}} = Slap.SlateDB.subscribe(db, :mine)
receive do
{:slap_slatedb_durable, ^ref, :mine, durable_seq} -> durable_seq
end