fix: Use thread-safe caches in apicontroller

[SystemKeeper]

We've seen crashes in background fetch lately, e.g.

Thread 2 name:
Thread 2 Crashed:
0   libswiftCore.dylib            	0x00000001918a9980 swift_isUniquelyReferenced_nonNull_native + 0 (SwiftObject.mm:1493)
1   NextcloudTalk                 	0x0000000100f83bb0 specialized Dictionary._Variant.isUniquelyReferenced() + 8 (/<compiler-generated>:0)
2   NextcloudTalk                 	0x0000000100f83bb0 specialized Dictionary._Variant.setValue(_:forKey:) + 8 (/<compiler-generated>:0)
3   NextcloudTalk                 	0x0000000100f83bb0 specialized Dictionary.subscript.setter + 8 (NCAPIController.swift:73)
4   NextcloudTalk                 	0x0000000100f83bb0 NCAPIController.getAPISessionManager(forAccountId:) + 596
5   NextcloudTalk                 	0x0000000100f861c8 NCAPIController.getRooms(forAccount:updateStatus:modifiedSince:completionBlock:) + 108 (NCAPIController.swift:291)
6   NextcloudTalk                 	0x000000010101142c NCRoomsManager.updateRooms(updatingUserStatus:onlyLastModified:withCompletion:) + 876 (NCRoomsManager.swift:151)
7   NextcloudTalk                 	0x0000000101010644 NCRoomsManager.updateRoomsAndChats(updatingUserStatus:onlyLastModified:withCompletionBlock:) + 880 (NCRoomsManager.swift:93)
8   NextcloudTalk                 	0x0000000101011048 @objc NCRoomsManager.updateRoomsAndChats(updatingUserStatus:onlyLastModified:withCompletionBlock:) + 132 (/<compiler-generated>:0)
9   NextcloudTalk                 	0x0000000100e72aa0 -[AppDelegate performBackgroundFetchWithCompletionHandler:] + 460 (AppDelegate.m:566)
10  NextcloudTalk                 	0x0000000100e72808 -[AppDelegate handleAppRefresh:] + 120 (AppDelegate.m:506)
11  BackgroundTasks               	0x00000001bf74ddbc __41-[BGTaskScheduler _runTask:registration:]_block_invoke_5 + 228 (BGTaskScheduler.m:662)

This boils down to parallel access to the swift dictionaries we used, so use NSCache, which is thread-safe.