Doubly skiplist

[v01dstar]

See previous incarnation https://github.com/tikv/rocksdb/pull/278

Add a doubly-linked skiplist implementation. So that, when traversing backward, each Prev() call is O(1) instead of O(logN).

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Connor1996, LykxSassinator

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Timeline:

• 2024-09-26 06:41:50.690800804 +0000 UTC m=+1720980.431224743: :ballot_box_with_check: agreed by LykxSassinator.
• 2024-09-26 07:28:51.141502425 +0000 UTC m=+1723800.881926364: :ballot_box_with_check: agreed by Connor1996.