fix: create single threaded executor for use with non-concurrent generators (#2236)

- Fixes #2228

worldedit-bukkit/src/main/java/com/fastasyncworldedit/bukkit/adapter/Regenerator.java

@@ -165,7 +165,11 @@ public abstract class Regenerator<IChunkAccess, ProtoChunk extends IChunkAccess,
                     .setNameFormat("fawe-regen-%d")
                     .build()
             );
-        } // else using sequential chunk generation, concurrent not supported
+        } else { // else using sequential chunk generation, concurrent not supported
+            executor = Executors.newSingleThreadExecutor(new ThreadFactoryBuilder()
+                    .setNameFormat("fawe-regen-%d")
+                    .build());
+        }
 
         //TODO: can we get that required radius down without affecting chunk generation (e.g. strucures, features, ...)?
         //for now it is working well and fast, if we are bored in the future we could do the research (a lot of it) to reduce the border radius