Add Notifier, implemented for PostgreSQL with LISTEN/NOTIFY

[mattn]

Problem

A relay running as several processes on one database (e.g. behind a load balancer) has no way to push an event accepted by one process to the clients subscribed on the others: live subscriptions live in each process's memory.

fiatjaf/relayer#167 added a Notifier interface on the relayer side and resolves it on the Relay first and on the Store second. This PR adds the store side, so a backend with native change notifications can provide it out of the box.

Change

type Notifier interface {
	Notify(context.Context, *nostr.Event) error
	Notifications(context.Context) (<-chan *nostr.Event, error)
}

Optional, like Counter. The relay calls Notify for every accepted event after saving it (ephemeral ones included, since they are never saved); Notifications must deliver everything any process notified, including the caller's own, and keep the channel open until ctx is done.

PostgresBackend implements it with pg_notify / LISTEN via pq.NewListener (which reconnects by itself):

• new NotifyChannel field, defaults to nostr_events
• events that fit in a pg_notify payload (8000 bytes) travel inline; bigger ones are announced by id and read back from the table by the receiver, so only ephemeral events above the limit are lost
• Notifications needs DatabaseURL since LISTEN takes over a dedicated connection

Tests

test/notify_test.go runs two backends on one embedded postgres and checks that a small event, an over-limit event (with full content read back) and an ephemeral event notified through one arrive on both, and that cancelling the context closes the channels.

Once released, relayer can switch to type Notifier = eventstore.Notifier.