The Worker and Dispatcher share the same poll loop logic
(Poller#start_loop) while having different functional requirements.

The Worker is poll looping despite not being able to execute new
jobs if at capacity.

The Dispatcher does require polling, but is reliant on shared
logic in Poller#start_loop for a Dispatcher specific optimization.

Changes:

Move the logic controlling the sleep interval per poll from
Poller#start_loop into Worker#poll and Dispatcher#poll by
requiring #poll to return the `delay` value passed into
interruptible_sleep.

Poller#start_loop:
* Removes the test based on the number of rows processed by #poll. This
  was Dispatcher specific logic.

Worker#poll:
* When Worker at full capacity: return a large value (10.minutes)
  effectively transforming Poller#start_loop from polling to wake-on-event.
* When Worker < capacity: return `polling_interval` and maintain the poll
  timing until ReadyExecutions become available.

Dispatcher#poll:
* When `due` ScheduledExecutions.zero? return `polling_interval` and
  maintain the existing poll timing when no ScheduledExecutions
  are available to process.
* When `due` ScheduledExecutions.postive? return 0. This results in
  interruptible_sleep(0) which returns immediately and without introducing
  any delays/sleeps between polls. This also allows for the existing behavior
  of looping through ScheduledExecutions via poll in order to check for
  shutdown requests between dispatch_next_batch interations.