Vary delays between attempts based on schedule

It must be possible to vary delay between job attempts based on schedule.

For example, if a task fails, 1st retry should be done after 10 seconds, 2nd one after 1 minute, 3rd one after 5 minutes, etc.

The patch adds support for specifying "retry_delay_schedule" for example as:

  'retry_delay_schedule' => '5,10,60,300',

(illustrating the scenario described above).

The same time delay could also be used multiple times - for example:

  'retry_delay_schedule' => '5*2,10*4,60*5,300',

would mean that:

• first 2 retries will be done after 5 second delay each
• next 4 retries will be done after 10 second delay each
• next 5 retries will be done after 60 second delay each
• all the remaining retries will be done after 5 minute delay each

If max attempts value is higher than number of defined retry_delay_schedule delays, the last delay value will be used for all the remaining attempts.

The change is 100% backwards compatible, and using 'retry_delay' => 5 still works as before. retry_delay_schedule takes precedence over retry_delay.