Feature/graphql download queue subscription send only updates (#1011)

* Emit only download changes instead of full status

The download subscription emitted the full download status, which, depending on how big the queue was, took forever because the graphql subscription does not support data loader batching, causing it to run into the n+1 problem

* Rename "DownloadManager#status" to "DownloadManager#updates"

* Add initial queue to download subscription type

Adds the current queue at the time of sending the initial message.
This field is null for all following messages after the initial one

* Optionally limit and omit download updates

To prevent the n+1 dataloader issue, the max number of updates included in the download subscription can be limited.
This way, the problem will be circumvented and instead, the latest download status should be (re-)fetched via the download status query, which does not run into this problem.

* Formatting

server/src/main/kotlin/suwayomi/tachidesk/graphql/queries/DownloadQuery.kt

@@ -1,6 +1,5 @@
 package suwayomi.tachidesk.graphql.queries
 
-import kotlinx.coroutines.flow.first
 import suwayomi.tachidesk.graphql.types.DownloadStatus
 import suwayomi.tachidesk.manga.impl.download.DownloadManager
 import suwayomi.tachidesk.server.JavalinSetup.future
@@ -9,6 +8,6 @@ import java.util.concurrent.CompletableFuture
 class DownloadQuery {
     fun downloadStatus(): CompletableFuture<DownloadStatus> =
         future {
-            DownloadStatus(DownloadManager.status.first())
+            DownloadStatus(DownloadManager.getStatus())
         }
 }