Centralize periodic checks for mutes, tempbans, vcalerts, reminders, and scheduled posts

This should result in a significant performance improvement.
The new method is also more precise than the old one, allowing
the aforementioned checks to be performed with second-precision.

backend/src/data/loops/upcomingRemindersLoop.ts

@@ -0,0 +1,67 @@
+import { lazyMemoize, MINUTES } from "../../utils";
+import moment from "moment-timezone";
+import { emitGuildEvent, hasGuildEventListener } from "../GuildEvents";
+import { Reminder } from "../entities/Reminder";
+import { Reminders } from "../Reminders";
+import Timeout = NodeJS.Timeout;
+
+const LOOP_INTERVAL = 15 * MINUTES;
+const MAX_TRIES_PER_SERVER = 3;
+const getRemindersRepository = lazyMemoize(() => new Reminders());
+const timeouts = new Map<number, Timeout>();
+
+function broadcastReminder(reminder: Reminder, tries = 0) {
+  if (!hasGuildEventListener(reminder.guild_id, "reminder")) {
+    // If there are no listeners registered for the server yet, try again in a bit
+    if (tries < MAX_TRIES_PER_SERVER) {
+      timeouts.set(
+        reminder.id,
+        setTimeout(() => broadcastReminder(reminder, tries + 1), 1 * MINUTES),
+      );
+    }
+    return;
+  }
+  emitGuildEvent(reminder.guild_id, "reminder", [reminder]);
+}
+
+export async function runUpcomingRemindersLoop() {
+  console.log("[REMINDERS LOOP] Clearing old timeouts");
+  for (const timeout of timeouts.values()) {
+    clearTimeout(timeout);
+  }
+
+  console.log("[REMINDERS LOOP] Setting timeouts for upcoming reminders");
+  const remindersDueSoon = await getRemindersRepository().getRemindersDueSoon(LOOP_INTERVAL);
+  for (const reminder of remindersDueSoon) {
+    const remaining = Math.max(0, moment.utc(reminder.remind_at!).diff(moment.utc()));
+    timeouts.set(
+      reminder.id,
+      setTimeout(() => broadcastReminder(reminder), remaining),
+    );
+  }
+
+  console.log("[REMINDERS LOOP] Scheduling next loop");
+  setTimeout(() => runUpcomingRemindersLoop(), LOOP_INTERVAL);
+}
+
+export function registerUpcomingReminder(reminder: Reminder) {
+  clearUpcomingReminder(reminder);
+
+  console.log("[REMINDERS LOOP] Registering new upcoming reminder");
+  const remaining = Math.max(0, moment.utc(reminder.remind_at!).diff(moment.utc()));
+  if (remaining > LOOP_INTERVAL) {
+    return;
+  }
+
+  timeouts.set(
+    reminder.id,
+    setTimeout(() => broadcastReminder(reminder), remaining),
+  );
+}
+
+export function clearUpcomingReminder(reminder: Reminder) {
+  console.log("[REMINDERS LOOP] Clearing upcoming reminder");
+  if (timeouts.has(reminder.id)) {
+    clearTimeout(timeouts.get(reminder.id)!);
+  }
+}