Refactor ConcurrencyLimit::WorkersMap into WorkerAttributes and fix gaps in workers with default concurrency limit

In gitlab-org/gitlab!194881 (diffs), we removed setting default concurrency_limit -> { 0 } to all workers in place of calculating a default limit based on percentage of sidekiq capacity gitlab-org/gitlab!194881 (diffs).

However, because we no longer set concurrency_limit -> { 0 } attribute explicitly, it created a gap in some places:

1. ConcurrencyLimitSampler no longer samples from all workers as @data no longer contains all workers.
2. ResumeWorker also only resumes workers in @data, even though workers with the default concurrency limit from the percentage-based are still subject to concurrency limiter.

This is also related to the discussion in gitlab-org/gitlab!194881 (comment 2620019914)

After this has been fixed, we should see the concurrency dashboard showing all workers.