concurrent locking is blocking execution

When running multiple `scalas` instances (installed using conscript as described on http://www.scala-sbt.org/0.13/docs/Scripts.html) all of the are hanging and just reporting:

```
Waiting for lock on /home/brandl/.conscript/boot/sbt.boot.lock to be available...
```

Locking should be optional or different instances should not interfere which each other.

通过运行多个使用 conscript 安装的 `scalas` 实例来重现问题，并观察它们在 `/home/brandl/.conscript/boot/sbt.boot.lock` 上的等待。首先定位管理此 boot lock 的代码，并确定并发实例的预期行为。当并发执行不再意外地相互干扰，并且锁定行为由可靠的检查覆盖时，即视为完成。

由索引模型根据 Issue 内容生成。