Add reply-from reply-modes to `ra:pipeline_command/4`

[the-mikedavis]

#314 adds new reply mode options for ra:process_command/3 which controls which member of the Ra cluster replies with gen_statem:reply/2 to a call:

• local - reply from a node local to the calling process
• {member, Member} - reply from the given member

which fall back to the leader if the given member is not in cluster membership. These are useful for blocking a caller until a command has been handled by the given member.

These should be added to ra:pipeline_command/4 as well to control which member of the cluster sends notifications. This would involve some refactoring of how notifications are stored within the cluster state (in ra_server_proc's #state.pending_notifys field) and the shape of the notify ra_server:effect():

ra/src/ra_server.erl

Line 153 in 09cc606

{notify, #{pid() => [term()]}} |

Currently, notifications are grouped by pid:

ra/src/ra_server.erl

Lines 2310 to 2321 in 09cc606

	add_reply(_, Reply, {notify, Corr, Pid},
	Effects, Notifys) ->
	% notify are casts and thus have to include their own pid()
	% reply with the supplied correlation so that the sending can do their
	% own bookkeeping
	CorrData = {Corr, Reply},
	case Notifys of
	#{Pid := T} ->
	{Effects, Notifys#{Pid => [CorrData | T]}};
	_ ->
	{Effects, Notifys#{Pid => [CorrData]}}
	end;

and replied to in batches:

ra/src/ra_server_proc.erl

Lines 1708 to 1731 in 09cc606

	send_applied_notifications(#state{pending_notifys = PendingNots} = State,
	Nots0) when map_size(PendingNots) > 0 ->
	Nots = ra_lib:maps_merge_with(fun(_K, V1, V2) ->
	V1 ++ V2
	end, PendingNots, Nots0),
	send_applied_notifications(State#state{pending_notifys = #{}}, Nots);
	send_applied_notifications(#state{} = State, Nots) ->
	Id = id(State),
	%% any notifications that could not be sent
	%% will be kept and retried
	RemNots = maps:filter(
	fun(Who, Correlations0) ->
	%% correlations are build up in reverse order so we need
	%% to reverse before sending
	Correlations = lists:reverse(Correlations0),
	ok =/= send_ra_event(Who, Correlations, Id,
	applied, State)
	end, Nots),
	case map_size(RemNots) of
	0 ->
	State;
	_ ->
	State#state{pending_notifys = RemNots}
	end.

This will need to be refactored so that notifications are grouped by pid and reply-from.