worker.process
所有的工作进程都是通过child_process.fork()来创建的，这个方法返回的对象被存储为.process。在工作进程中， process属于全局对象。
需要注意：当process上发生 'disconnect'事件，并且.exitedAfterDisconnect的值不是true时，工作进程会调用 process.exit(0)。这样就可以防止连接意外断开。
All workers are created using child_process.fork(), the returned object
from this function is stored as .process. In a worker, the global process
is stored.
See: Child Process module.
Note that workers will call process.exit(0) if the 'disconnect' event occurs
on process and .exitedAfterDisconnect is not true. This protects against
accidental disconnection.