Improvements for Job#retry

[xdc0]

This PR makes some improvements on the current Job#retry implementation:

• The retry operation is now implemented through a script, to follow suit with the other operations
• The retry operation does several checks while attempting to retry a job:
  • It checks if the job exists, so that a non-existent job can be accidentally retried
  • It checks if the job is currently locked, to avoid possible corruption on job data
  • It checks if the job is actually failed, so it avoids retrying job that are in any other state that is not failed.

[xdc0]

This is similar to: #232 except that it has more extensive failure handling, more job integrity checks, is in scripts.js and has tests

[manast]

For simplicity here we could just throw and error or not returning anything, and skip the jobDone callback.

[xdc0]

Ah thanks! will amend

[manast]

instead of this nested code I would propose something like this:

if ((redis.call("EXISTS", KEYS[1]) ~= 1) then
  return 0
end
if ((redis.call("EXISTS", KEYS[2])  ~= 0) then
  return -1
end
.
.
.
return 1;