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.

[rosa]

Hey @cupatea, thank you so much and sorry for the delay! I'm finally catching up with PRs.

This one was on purpose, though. The public, documented API to manage dynamic recurring tasks is:

SolidQueue.schedule_recurring_task(..)
SolidQueue.unschedule_recurring_task(..)

That is, to update a dynamic task, you need to unschedule and schedule it again. This is to keep the code simpler.
Because of this, I'm closing this PR, but thank you so much in any case!