Use dedicate operation executor

app/src/main/java/eu/faircode/email/Helper.java

@@ -260,9 +260,6 @@ public class Helper {
     private static ExecutorService sMediaExecutor = null;
     private static ExecutorService sDownloadExecutor = null;
 
-    private static int sOperationIndex = 0;
-    private static final ExecutorService[] sOperationExecutor = new ExecutorService[OPERATION_WORKERS];
-
     static ExecutorService getSerialExecutor() {
         if (sSerialExecutor == null)
             sSerialExecutor = getBackgroundExecutor(1, "serial");
@@ -293,16 +290,6 @@ public class Helper {
         return sDownloadExecutor;
     }
 
-    static ExecutorService getOperationExecutor() {
-        synchronized (sOperationExecutor) {
-            if (sOperationExecutor[sOperationIndex] == null)
-                sOperationExecutor[sOperationIndex] = getBackgroundExecutor(1, "operation");
-            ExecutorService result = sOperationExecutor[sOperationIndex];
-            sOperationIndex = (sOperationIndex + 1) % sOperationExecutor.length;
-            return result;
-        }
-    }
-
     static ExecutorService getBackgroundExecutor(int threads, final String name) {
         ThreadFactory factory = new ThreadFactory() {
             private final AtomicInteger threadId = new AtomicInteger();

app/src/main/java/eu/faircode/email/ServiceSynchronize.java

@@ -1550,7 +1550,7 @@ public class ServiceSynchronize extends ServiceBase implements SharedPreferences
                 Log.i(account.name + " run thread=" + currentThread);
 
                 final ObjectHolder<TwoStateOwner> cowner = new ObjectHolder<>();
-                final ExecutorService executor = Helper.getOperationExecutor();
+                final ExecutorService executor = Helper.getBackgroundExecutor(1, "operation." + account.id);
 
                 // Debug
                 SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(this);