Add Command: "wait <id>" that responds when a job is deleted

Add a command: `wait <id>\r\n` that responds with `DELETED <id>\r\n` or `BURIED <id>\r\n` only once a job has been deleted or buried.

This allows a client to be notified about the status of a job without polling.

### From the docs ###
The wait command waits until a particular job is out of the system before responding. It can be used to block a client until a job is completed. It looks like this:

```
wait <id>\r\n
```

##### `wait` options

* `<id>` is the job id to wait for.

##### `wait` responses

The client expects one line of response once the state of the job has changed, which may be:

* `DELETED <id>\r\n` to indicate the job was deleted via the `delete` command (normal success).
* `BURIED <id>\r\n` to indicate the job was buried via the `bury` command.
* `NOT_FOUND\r\n` if the job id does not exist.

A response won't be sent by the server until the specified job is deleted or buried. The wait command can be issued multiple times before a response is received in order to wait on a collection of jobs.