Reimplement locking w/o relying on hostInfo cmd

Because the hostInfo MongoDB command may not be allowed in some
environments. Also, in sharded deployments the clock skew may differ
between different shards so using hostInfo is not enough. This new
implementation simply assumes the clocks are in sync within a certain
range and throws an error if clocks are found to be out of sync.

lib/local-cache.ts

@@ -398,7 +398,7 @@ async function refresh(): Promise<void> {
     return;
   }
 
-  const unlockOrExtend = await cache.lock("presets_hash_lock", 3);
+  const lockToken = await cache.acquireLock("presets_hash_lock", 4000, 4000);
 
   const res = await Promise.all([
     fetchPresets(),
@@ -432,7 +432,7 @@ async function refresh(): Promise<void> {
   currentSnapshot = computeHash(snapshot);
   snapshots.set(currentSnapshot, snapshot);
   await cache.set("presets_hash", currentSnapshot, 300);
-  await unlockOrExtend(0);
+  await cache.releaseLock("presets_hash_lock", lockToken);
 
   nextRefresh = now + (REFRESH - (now % REFRESH));
 }