Remove caching of tasks, faults and operations

No longer seems to provide significant performance benefit to justify the
drawback of the added code complexity and the additional DB write query.

lib/types.ts

@@ -160,7 +160,6 @@ export interface SessionContext {
   authState: number;
   tasks?: Task[];
   operations?: { [commandKey: string]: Operation };
-  cacheUntil?: number;
   syncState?: SyncState;
   lastActivity?: number;
   extendLock?: number;