Decouple Cache and Download behaviour (#493)

* Separate cache dir from download dir

* Move downloader logic outside of caching/image download logic

* remove unnecessary method duplication

* moved download logic inside download provider

* optimize and handle partial downloads

* made code review changes

server/src/main/kotlin/suwayomi/tachidesk/manga/impl/ChapterDownloadHelper.kt

@@ -0,0 +1,32 @@
+package suwayomi.tachidesk.manga.impl
+
+import kotlinx.coroutines.CoroutineScope
+import suwayomi.tachidesk.manga.impl.download.DownloadedFilesProvider
+import suwayomi.tachidesk.manga.impl.download.FolderProvider
+import suwayomi.tachidesk.manga.impl.download.model.DownloadChapter
+import java.io.InputStream
+
+object ChapterDownloadHelper {
+    fun getImage(mangaId: Int, chapterId: Int, index: Int): Pair<InputStream, String> {
+        return provider(mangaId, chapterId).getImage(index)
+    }
+
+    fun delete(mangaId: Int, chapterId: Int): Boolean {
+        return provider(mangaId, chapterId).delete()
+    }
+
+    suspend fun download(
+        mangaId: Int,
+        chapterId: Int,
+        download: DownloadChapter,
+        scope: CoroutineScope,
+        step: suspend (DownloadChapter?, Boolean) -> Unit
+    ): Boolean {
+        return provider(mangaId, chapterId).download(download, scope, step)
+    }
+
+    // return the appropriate provider based on how the download was saved. For the logic is simple but will evolve when new types of downloads are available
+    private fun provider(mangaId: Int, chapterId: Int): DownloadedFilesProvider {
+        return FolderProvider(mangaId, chapterId)
+    }
+}