Make offline checking intervals configurable

Fixes #3855

server/lib/offlineThresholds.ts

@@ -0,0 +1,26 @@
+export type OfflineThresholds = {
+    // how often the offline checkers run
+    checkIntervalMs: number;
+    // newt sites that stop pinging for this long are marked offline
+    newtMs: number;
+    // wireguard sites with no bandwidth update for this long are marked offline
+    newtWireguardMs: number;
+    // olm clients that stop pinging for this long are marked offline
+    olmMs: number;
+    // remote exit nodes that stop pinging for this long are marked offline
+    remoteExitNodeMs: number;
+};
+
+export const DEFAULT_OFFLINE_THRESHOLDS: OfflineThresholds = {
+    checkIntervalMs: 30 * 1000, // 30 seconds
+    newtMs: 2 * 60 * 1000, // 2 minutes
+    newtWireguardMs: 8 * 60 * 1000, // 8 minutes
+    olmMs: 2 * 60 * 1000, // 2 minutes
+    remoteExitNodeMs: 2 * 60 * 1000 // 2 minutes
+};
+
+// The thresholds are only configurable in the private config file, so the
+// open build always uses the defaults. See server/private/lib/offlineThresholds.ts
+export function getOfflineThresholds(): OfflineThresholds {
+    return DEFAULT_OFFLINE_THRESHOLDS;
+}