Handle rollback during postgres peer election

Infrequently, the postgres peer election transaction returns `{:error,
:rollback}`. Now that return value is handled to prevent a match error.

The peer maintains its current `leader?` status on rollback—this may
cause inconsistency if the leader encounters an error and multiple
rollbacks happen in sequence. That tradeoff is acceptable because the
situation is unlikely and less of an issue than crashing the peer.

Closes #1007

lib/oban/peers/postgres.ex

@@ -90,14 +90,23 @@ defmodule Oban.Peers.Postgres do
 
     state =
       :telemetry.span([:oban, :peer, :election], meta, fn ->
-        {:ok, state} =
-          Repo.transaction(state.conf, fn ->
-            state
-            |> delete_expired_peers()
-            |> upsert_peer()
-          end)
-
-        {state, %{meta | leader: state.leader?}}
+        fun = fn ->
+          state
+          |> delete_expired_peers()
+          |> upsert_peer()
+        end
+
+        case Repo.transaction(state.conf, fun) do
+          {:ok, state} ->
+            {state, %{meta | leader: state.leader?}}
+
+          {:error, :rollback} ->
+            # The peer maintains its current `leader?` status on rollback—this may cause
+            # inconsistency if the leader encounters an error and multiple rollbacks happen in
+            # sequence. That tradeoff is acceptable because the situation is unlikely and less of
+            # an issue than crashing the peer.
+            {state, meta}
+        end
       end)
 
     {:noreply, schedule_election(state)}