Speed up counsel-yank-pop on large kill-ring

[basil-conto]

cl-delete-duplicates rapidly slows down on large inputs. Alternatives to the approach in this patch:

• Avoid deduplication beyond a certain threshold.
• Deduplicate only when kill-do-not-save-duplicates is non-nil.
• Make equality test customizable.
• Ignore text properties by default.

Fixes #3040.