Fix asyncCatcher on hardcrash shutdown

[spring-dependency-management]

Paper used to have a flag for AsyncCatcher called enabled, which would be toggled false when shutting the server down during a hard crash as seen here.

       logger.log(Level.SEVERE, "--- DO NOT REPORT THIS TO PAPER - THIS IS NOT A BUG OR A CRASH ---");
                }

                logger.log(Level.SEVERE, "------------------------------");

                if (isLongTimeout) {
                    if (!server.hasStopped()) {
                        AsyncCatcher.enabled = false; // Disable async catcher incase it interferes with us
                        server.forceTicks = true;
                        if (this.restart) {
                            RestartCommand.addShutdownHook(SpigotConfig.restartScript);
                        }

The reason for this flag is without disabling the main thread checker, shutdown from a thread other than main will obviously fail due to the checks everywhere.

Paper's chunk changes broke this flags functionality, i.e it is set but never actually checked anywhere, this PR re-adds it being checked.

Fixes #12030

[lynxplay]

Closed by leaf in a392d47