Explicitly release server.lock file when shutdown the server. (#3619)

Previously, this relied on PHP itself to release locks during the resource destructor phase during process exit, but sometimes it doesn't for god knows what reason. This change makes the lock file get explicitly released before the process dies.

tests/phpstan/configs/l7-baseline.neon

@@ -82,7 +82,7 @@ parameters:
 
 		-
 			message: "#^Parameter \\#1 \\$fp of function flock expects resource, resource\\|false given\\.$#"
-			count: 3
+			count: 4
 			path: ../../../src/pocketmine/PocketMine.php
 
 		-
@@ -105,6 +105,11 @@ parameters:
 			count: 1
 			path: ../../../src/pocketmine/PocketMine.php
 
+		-
+			message: "#^Parameter \\#1 \\$fp of function fclose expects resource, resource\\|false given\\.$#"
+			count: 1
+			path: ../../../src/pocketmine/PocketMine.php
+
 		-
 			message: "#^Parameter \\#1 \\$buffer of method pocketmine\\\\nbt\\\\NBTStream\\:\\:readCompressed\\(\\) expects string, string\\|false given\\.$#"
 			count: 1