Do not use persistent term

If a repository may be shut on and off, cleaning up persistent
term can be too expensive.

lib/ecto/repo/registry.ex

@@ -18,13 +18,9 @@ defmodule Ecto.Repo.Registry do
   end
 
   def lookup(repo) when is_atom(repo) do
-    case Process.whereis(repo) do
-      nil ->
-        raise "could not lookup Ecto repo #{inspect(repo)} because it was not started or it does not exist"
-
-      _ ->
-        :persistent_term.get(repo, nil)
-    end
+    GenServer.whereis(repo)
+    |> Kernel.||(raise "could not lookup Ecto repo #{inspect repo} because it was not started or it does not exist")
+    |> lookup()
   end
 
   def lookup(pid) when is_pid(pid) do
@@ -42,19 +38,13 @@ defmodule Ecto.Repo.Registry do
   @impl true
   def handle_call({:associate, pid, name, value}, _from, table) do
     ref = Process.monitor(pid)
-    name && :persistent_term.put(name, value)
     true = :ets.insert(table, {pid, ref, name, value})
     {:reply, :ok, table}
   end
 
   @impl true
   def handle_info({:DOWN, ref, _type, pid, _reason}, table) do
-    [{^pid, ^ref, name, _}] = :ets.lookup(table, pid)
-    # We don't delete from persistent term on purpose. Since the process is
-    # named, we can assume it does not start dynamically, so it will either
-    # restart or the amount of memory it uses is negligibla to justify the
-    # process purging done by persistent_term. If the repo is restarted and
-    # stores the same metadata, then no purging happens either.
+    [{^pid, ^ref, _, _}] = :ets.lookup(table, pid)
     :ets.delete(table, pid)
     {:noreply, table}
   end