Implement peak commands

[vimukthi-git]

The peek commands let the client inspect a job in the system. There are four
variations. All but the first operate only on the currently used tube.

• "peek \r\n" - return job .

• "peek-ready\r\n" - return the next ready job.

• "peek-delayed\r\n" - return the delayed job with the shortest delay left.

• "peek-buried\r\n" - return the next job in the list of buried jobs.

There are two possible responses, either a single line:

• "NOT_FOUND\r\n" if the requested job doesn't exist or there are no jobs in
  the requested state.

Or a line followed by a chunk of data, if the command was successful:

FOUND <id> <bytes>\r\n
<data>\r\n

• is the job id.

• is an integer indicating the size of the job body, not including
  the trailing "\r\n".

• is the job body -- a sequence of bytes of length from the
  previous line.