Revert "Revert "Merge pull request #3216 from rabbitmq/mk-quorum-queu…

…e-oldest-known-message-command""

This reverts commit e3afd35.

deps/rabbit/src/rabbit_fifo.erl

@@ -800,6 +800,19 @@ handle_aux(_RaState, cast, tick, #aux{name = Name,
 handle_aux(_RaState, cast, eol, #aux{name = Name} = Aux, Log, _) ->
     ets:delete(rabbit_fifo_usage, Name),
     {no_reply, Aux, Log};
+handle_aux(_RaState, {call, _From}, oldest_entry_timestamp, Aux,
+           Log, #?MODULE{ra_indexes = Indexes}) ->
+    Ts = case rabbit_fifo_index:smallest(Indexes) of
+        %% if there are no entries, we return current timestamp
+        %% so that any previously obtained entries are considered older than this
+        undefined ->
+            erlang:system_time(millisecond);
+        Idx when is_integer(Idx) ->
+            {{_, _, {_, Meta, _, _}}, _Log1} = ra_log:fetch(Idx, Log),
+            #{ts := Timestamp} = Meta,
+           Timestamp
+    end,
+    {reply, {ok, Ts}, Aux, Log};
 handle_aux(_RaState, {call, _From}, {peek, Pos}, Aux0,
            Log0, MacState) ->
     case rabbit_fifo:query_peek(Pos, MacState) of