Add seshat:fold/3 to fold over all ids in a group

[the-mikedavis]

This can be used to efficiently query for specific counters across all ids in a group. Equivalently you could use counters/1 and then maps:fold/3, however this counters/1 creates a map of maps with all counters - this introduces a lot of intermediary garbage.

[the-mikedavis]

I was imagining that this could be nice for rabbitmq/rabbitmq-server#15098. With rabbitmq/osiris#197 we could efficiently find all writers with replicas that are behind, something like:

seshat:fold(
  fun ({osiris_writer, SQ}, CRef, Acc) ->
          case counters:get(CRef, 9) > MaxDiff of
              true ->
                  [SQ | Acc];
              false ->
                  Acc
          end;
      (_, _, Acc) ->
          Acc
  end, [], osiris).

[the-mikedavis]

API-wise, passing the counters ref feels wrong. Ideally we could take advantage of the #entry.field_spec here so that the fold function could use the field name atoms instead of indexes.

Maybe we can pass in a function? Something like:

fold(Fun, Acc0, Group) ->
    ets:foldl(fun(#entry{id = Id,
                         field_spec = FieldsSpec,
                         cref = CRef}, Acc) ->
                      Fields = resolve_fields_spec(FieldsSpec),
                      GetCounterFun = fun(Field) ->
                                          %% lists:keyfind/3 the field and use
                                          %% its index with counters:get/2
                                      end,
                      Fun(Id, GetCounterFun, Acc)
              end, Acc0, seshat_counters_server:get_table(Group)).

[the-mikedavis]

Also - seems like it might be valuable to make fields_spec() a map keyed by name? I think that would benefit a few of the functions in this module