Add per-arena setting clear-wave-leeway.

Introduces a new arena setting that changes the "emptiness check" for 
`clear-wave-before-next`, `clear-wave-before-boss`, and the final wave 
check, allowing for a customizable _leeway_.

By default, the leeway is 0, which means the monster set has to be
completely empty for the checks to pass. With a value of 2, the set
may contain up to two monsters for the checks to pass, and so on.

The leeway should help alleviate some of the issues some people have
with their arena sessions when monsters "disappear" behind terrain or
end up in hard-to-reach areas. This is by no means a real "solution"
to the underlying problem, since the build-up of monsters in longer
sessions will just result in the issue being pushed to later waves.

We'll see if the setting leaves any additional customization to be
desired and perhaps defer any such requests to the Sessions Rework.

Closes #706

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

@@ -42,6 +42,7 @@ public class MASpawnThread implements Runnable
 
     private int playerCount, monsterLimit;
     private boolean waveClear, bossClear, preBossClear, wavesAsLevel;
+    private int clearLeeway;
     private int waveInterval;
     private int nextWaveDelay;
 
@@ -76,6 +77,7 @@ public class MASpawnThread implements Runnable
         waveClear = arena.getSettings().getBoolean("clear-wave-before-next", false);
         bossClear = arena.getSettings().getBoolean("clear-boss-before-next", false);
         preBossClear = arena.getSettings().getBoolean("clear-wave-before-boss", false);
+        clearLeeway = arena.getSettings().getInt("clear-wave-leeway", 0);
         wavesAsLevel = arena.getSettings().getBoolean("display-waves-as-level", false);
         waveInterval = arena.getSettings().getInt("wave-interval", 3);
         nextWaveDelay = arena.getSettings().getInt("next-wave-delay", 0);
@@ -308,18 +310,18 @@ public class MASpawnThread implements Runnable
             return false;
         }
 
-        // Check for wave and pre boss clear
-        if (waveClear && !monsterManager.getMonsters().isEmpty()) {
+        // Check for wave clear
+        if (waveClear && monsterManager.getMonsters().size() > clearLeeway) {
             return false;
         }
 
         // Check for pre boss clear
-        if (preBossClear && waveManager.getNext().getType() == WaveType.BOSS && !monsterManager.getMonsters().isEmpty()) {
+        if (preBossClear && waveManager.getNext().getType() == WaveType.BOSS && monsterManager.getMonsters().size() > clearLeeway) {
             return false;
         }
 
         // Check for final wave
-        if (!monsterManager.getMonsters().isEmpty() && waveManager.getWaveNumber() == waveManager.getFinalWave()) {
+        if (monsterManager.getMonsters().size() > clearLeeway && waveManager.getWaveNumber() == waveManager.getFinalWave()) {
             return false;
         }
 

src/main/resources/res/settings.yml

@@ -7,6 +7,7 @@ default-class: ''
 clear-wave-before-next: false
 clear-boss-before-next: false
 clear-wave-before-boss: false
+clear-wave-leeway: 0
 soft-restore: false
 soft-restore-drops: false
 require-empty-inv-join: false