Detect updates to dynamic recurring tasks on poll

[cupatea]

Fixes #738

Scheduler::RecurringSchedule#reschedule_dynamic_tasks previously only detected created and deleted dynamic tasks. Updates to an existing task's were ignored, so the scheduler kept running against the pre-update version until restart.

Approach:

Each entry in scheduled_tasks is now a ScheduledRecurringTask struct that pairs the Concurrent::ScheduledTask handle with the source RecurringTask AR record:

ScheduledRecurringTask = Struct.new(:task, :handle) do
  delegate :key, :updated_at, to: :task
  delegate :cancel, to: :handle
end

Add new stepreschedule_dynamic_tasks that replaces any tasks with updated_at differs from the cached ones.

[cupatea]

Hey @rosa, during release testing with Solid Queue dynamic tasks (#553), QA identified an issue that seems to have been missed in the initial implementation.
We’ve applied the same fix as a patch on our side.