Add per-arena setting auto-leave-on-end.

Introduces a new arena setting to force spectators to leave the arena
when the current session ends. If set to `true`, it "overrides" the
behavior of `spectate-on-death: true` when players respawn, such that
they only become spectators if there is an active arena session.

The respawn behavior technically means that it is possible for a player
to begin spectating "the next session" if a new session begins between
them dying and clicking Respawn on the death screen. Ideally, we would
want the player to auto-leave, because the session _they_ participated
in ended, but we don't have a concept of "previous sessions", so this
quirky behavior will have to do.

Closes #682

src/main/java/com/garbagemule/MobArena/ArenaImpl.java

@@ -643,6 +643,11 @@ public class ArenaImpl implements Arena
         // Restore enabled status.
         enabled = en;
 
+        // Auto-leave
+        if (settings.getBoolean("auto-leave-on-end", false)) {
+            specPlayers.forEach(this::playerLeave);
+        }
+
         return true;
     }
 
@@ -903,11 +908,19 @@ public class ArenaImpl implements Arena
         specPlayers.add(p);
 
         if (settings.getBoolean("spectate-on-death", true)) {
-            messenger.tell(p, Msg.SPEC_PLAYER_SPECTATE);
-        } else {
-            plugin.getServer().getScheduler()
-                .scheduleSyncDelayedTask(plugin, () -> playerLeave(p));
+            // At this point, we know that we want players to become
+            // spectators when they respawn, but if we also want them
+            // to auto-leave on arena end, we first need to make sure
+            // the arena is running. If not, the session has probably
+            // ended, so we fall through to schedule an auto-leave.
+            if (!settings.getBoolean("auto-leave-on-end", false) || running) {
+                messenger.tell(p, Msg.SPEC_PLAYER_SPECTATE);
+                return;
+            }
         }
+
+        plugin.getServer().getScheduler()
+            .scheduleSyncDelayedTask(plugin, () -> playerLeave(p));
     }
 
     @Override

src/main/resources/res/settings.yml

@@ -15,6 +15,7 @@ pvp-enabled: false
 monster-infight: false
 allow-teleporting: false
 spectate-on-death: true
+auto-leave-on-end: false
 share-items-in-arena: true
 min-players: 0
 max-players: 0