[5.8] Job Based Retry Delay

[taylorotwell]

This PR implements support for job based retry delays. Previously, this was only determined at a worker level via the --delay option on the worker command. However, sometimes jobs need to have different failure retry rates for various reasons, such as third-party API limits, etc. This is also in line with allowing job based maximum retries and timeout values.

This should allow queued jobs to determine their own retry delay when necessary. This may be done using a simple retryAfter property (integer - seconds) on the job class.

public $retryAfter = 10;

Or, you may define a retryAfter method if more complex logic is required to determine the retry delay. The method may return an integer or a DateTime. If a timestamp is returned, the seconds until that DateTime is used as the delay (e.g. return now()->addSeconds(5) to delay 5 seconds).

Closes laravel/ideas#537