increase shell lifetime to 1s after death of target

Problem: two warship fight each other, if one back off he won't be hit
by the ennemy at all. he'll kill other warship, then all shells
dissapear so 0% damage are received. It's OP and unfair.
Solution: make warship's shells last 2s after its death so even if one
warship backs off he'll eventually receive some damages. 2s is enough to
get between 250-750 HP loss

src/core/configuration/Config.ts

@@ -88,6 +88,7 @@ export interface Config {
   maxPopulation(player: Player | PlayerView): number;
   cityPopulationIncrease(): number;
   boatAttackAmount(attacker: Player, defender: Player | TerraNullius): number;
+  warshipShellLifetime(): number;
   boatMaxNumber(): number;
   allianceDuration(): Tick;
   allianceRequestCooldown(): Tick;

src/core/configuration/DefaultConfig.ts

@@ -470,6 +470,10 @@ export class DefaultConfig implements Config {
     return Math.floor(attacker.troops() / 5);
   }
 
+  warshipShellLifetime(): number {
+    return 20; // in ticks (one tick is 100ms)
+  }
+
   attackAmount(attacker: Player, defender: Player | TerraNullius) {
     if (attacker.type() == PlayerType.Bot) {
       return attacker.troops() / 20;

src/core/execution/ShellExecution.ts

@@ -8,6 +8,8 @@ export class ShellExecution implements Execution {
   private active = true;
   private pathFinder: PathFinder;
   private shell: Unit;
+  private mg: Game;
+  private destroyAtTick: number = -1;
 
   constructor(
     private spawn: TileRef,
@@ -18,6 +20,7 @@ export class ShellExecution implements Execution {
 
   init(mg: Game, ticks: number): void {
     this.pathFinder = PathFinder.Mini(mg, 2000, true, 10);
+    this.mg = mg;
   }
 
   tick(ticks: number): void {
@@ -30,13 +33,19 @@ export class ShellExecution implements Execution {
     }
     if (
       !this.target.isActive() ||
-      !this.ownerUnit.isActive() ||
-      this.target.owner() == this.shell.owner()
+      this.target.owner() == this.shell.owner() ||
+      (this.destroyAtTick != -1 && this.mg.ticks() >= this.destroyAtTick)
     ) {
       this.shell.delete(false);
       this.active = false;
       return;
     }
+
+    if (this.destroyAtTick == -1 && !this.ownerUnit.isActive()) {
+      this.destroyAtTick =
+        this.mg.ticks() + this.mg.config().warshipShellLifetime();
+    }
+
     for (let i = 0; i < 3; i++) {
       const result = this.pathFinder.nextTile(
         this.shell.tile(),