Merge bitcoin/bitcoin#30286: cluster mempool: optimized candidate search

9ad2fe7 clusterlin: only start/use search when enough iterations left (Pieter Wuille)
bd04435 clusterlin: improve heuristic to decide split transaction (optimization) (Pieter Wuille)
71f2629 clusterlin: include topological pot subsets automatically (optimization) (Pieter Wuille)
e20fda7 clusterlin: reduce computation of unnecessary pot sets (optimization) (Pieter Wuille)
6060a94 clusterlin bench: add example hard cluster benchmarks (Pieter Wuille)
2965fbf clusterlin: track upper bound potential set for work items (optimization) (Pieter Wuille)
9e43e4c clusterlin: use feerate-sorted depgraph in SearchCandidateFinder (Pieter Wuille)
b80e6df clusterlin: add reordering support for DepGraph (Pieter Wuille)
85a285a clusterlin: separate initial search entries per component (optimization) (Pieter Wuille)
e4faea9 clusterlin bench: have low/high iter benchmarks instead of per-iter (Pieter Wuille)

Pull request description:

  Part of cluster mempool: #30289

  Depends on #30126, and was split off from it.

  This improves the candidate search algorithm introduced in the previous PR with a variety of optimizations.

  The resulting search algorithm largely follows Section 2 of [How to linearize your cluster](https://delvingbitcoin.org/t/how-to-linearize-your-cluster/303#h-2-finding-high-feerate-subsets-5), though with a few changes:
  * Connected component analysis is performed inside the search algorithm (creating initial work items per component for each candidate), rather than once at a higher level. This duplicates some work but is significantly simpler in implementation.
  * No ancestor-set based presplitting inside the search is performed; instead, the `best` value is initialized with the best topologically valid set known to the LIMO algorithm before search starts: the better one out of the highest-feerate remaining ancestor set, and the highest-feerate prefix of remaining transactions in `old_linearization`.
  * Work items are represented using an included set *inc* and an undefined set *und*, rather than included and excluded.
  * Potential sets *pot* are not computed for work items with empty *inc*.

  At a high level, the only missing optimization from that post is bottleneck analysis; my thinking is that it only really helps with clusters that are already relatively cheap to linearize (doing so would need to be done at a higher level, not inside the search algorithm).

  ---

  Overview of the impact of each commit here on linearize performance:
  * **[clusterlin bench: have low/high iter benchmarks instead of per-iter](https://github.com/bitcoin/bitcoin/pull/30286/commits/21a184db63bd13cf0c2dff9e6e61ca85f2ff4454)**: no impact
  * **[separate initial search entries per component (optimization)](https://github.com/bitcoin/bitcoin/pull/30286/commits/c84c5c86ba6c12ce11e997d0c59d60694c667488)**: reduce iterations, increase start-up cost
  * **[add reordering support for DepGraph](https://github.com/bitcoin/bitcoin/pull/30286/commits/019ff2960976b677d91bab47e75be6ed180b37a3)**: no impact
  * **[use feerate-sorted depgraph in SearchCandidateFinder](https://github.com/bitcoin/bitcoin/pull/30286/commits/8e27dd5a220237da5d362376db6406e47f063776)**: typically reduce iterations, increase start-up cost
  * **[track upper bound potential set for work items](https://github.com/bitcoin/bitcoin/pull/30286/commits/781e0fb3aa5cbd0983b01547481eefa0804dfb5b)**: reduce iterations, increase cost per iteration
  * **[reduce computation of unnecessary pot sets](https://github.com/bitcoin/bitcoin/pull/30286/commits/9fe834fa979c8ab038aec57dac2f468ca70c41a9)**: reduce cost per iteration
  * **[include topological pot subsets automatically](https://github.com/bitcoin/bitcoin/pull/30286/commits/30612710a4e5fc8c950bd1ff4ab9fa843b59132d)**: reduce iterations, increase cost per iteration
  * **[improve heuristic to decide split transaction](https://github.com/bitcoin/bitcoin/pull/30286/commits/1880c00ab1661dbfc867f69e7556b28bc3cf7a42)**: typically reduce iterations, increase cost per iteration
  * **[only start/use search when enough iterations left](https://github.com/bitcoin/bitcoin/pull/30286/commits/12760a57b3a6cd1aeb3b7532311f648de2b45aa4)**: just account for start-up cost as equivalent iterations

ACKs for top commit:
  sdaftuar:
    ACK 9ad2fe7
  instagibbs:
    reACK 9ad2fe7
  glozow:
    reACK 9ad2fe7, just have a question about the docs

Tree-SHA512: 108bcbb0676f36071eb83954059b5f3d6646c745015b644a2a5d7f5a8ac9424c2d01d339fa6318a3aff4cf313308e85bb80b0090899720a3fcba027b8025590a

Author: glozow