Stale metadata in the cache

[mdrlzy]

Right now we are not removing lost resource meta, which causes npe:

06-09 00:56:01.152 E/ACRA    (16329): ACRA caught a NullPointerException for space.taran.arknavigator
06-09 00:56:01.152 E/ACRA    (16329): java.lang.NullPointerException
06-09 00:56:01.152 E/ACRA    (16329): 	at space.taran.arklib.domain.preview.RootPreviewProcessor.initKnownResources(RootPreviewProcessor.kt:128)
06-09 00:56:01.152 E/ACRA    (16329): 	at space.taran.arklib.domain.preview.RootPreviewProcessor.access$initKnownResources(RootPreviewProcessor.kt:15)
06-09 00:56:01.152 E/ACRA    (16329): 	at space.taran.arklib.domain.preview.RootPreviewProcessor$init$2.invokeSuspend(RootPreviewProcessor.kt:44)
06-09 00:56:01.152 E/ACRA    (16329): 	at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
06-09 00:56:01.152 E/ACRA    (16329): 	at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:106)
06-09 00:56:01.152 E/ACRA    (16329): 	at kotlinx.coroutines.scheduling.CoroutineScheduler.runSafely(CoroutineScheduler.kt:570)
06-09 00:56:01.152 E/ACRA    (16329): 	at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.executeTask(CoroutineScheduler.kt:750)
06-09 00:56:01.152 E/ACRA    (16329): 	at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.runWorker(CoroutineScheduler.kt:677)
06-09 00:56:01.152 E/ACRA    (16329): 	at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.run(CoroutineScheduler.kt:664)
06-09 00:56:01.152 E/ACRA    (16329): 	Suppressed: kotlinx.coroutines.DiagnosticCoroutineContextException: [StandaloneCoroutine{Cancelling}@a414215, Dispatchers.Default]

    private suspend fun initKnownResources() {
        _busy.emit(true)
        metadata.state().forEach { (id, meta) ->
            val path = index.getPath(id)!!
            generate(id, path, meta)
        }
    ...
    }

RootPreviewProcessor takes meta, but there is no such id in index, because resource was deleted, but meta was not deleted
See ARK-Builders/arklib#40