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This PR addresses several optimization points for TxIndex.Search:

1. Remove expensive map access from sorting function
2. Replace sort.Slice to sort.Sort with sorter approach
3. Reduce heap allocations from extractEventSeqFromKey

benchstat result:
```
goos: darwin
goarch: amd64
pkg: github.com/cometbft/cometbft/state/txindex/kv
cpu: Intel(R) Core(TM) i9-10910 CPU @ 3.60GHz
                     │   old.txt    │              new.txt               │
                     │    sec/op    │   sec/op     vs base               │
TxSearchBigResult-20   3236.1m ± 7%   945.9m ± 9%  -70.77% (p=0.002 n=6)

                     │   old.txt    │               new.txt               │
                     │     B/op     │     B/op      vs base               │
TxSearchBigResult-20   426.1Mi ± 0%   311.6Mi ± 0%  -26.86% (p=0.002 n=6)

                     │   old.txt   │              new.txt               │
                     │  allocs/op  │  allocs/op   vs base               │
TxSearchBigResult-20   4.040M ± 0%   2.040M ± 0%  -49.51% (p=0.002 n=6)
```

#### PR checklist

- [x] Tests written/updated
- [x] Changelog entry added in `.changelog` (we use
[unclog](https://github.com/informalsystems/unclog) to manage our
changelog)
- [x] Updated relevant documentation (`docs/` or `spec/`) and code
comments
- [x] Title follows the [Conventional
Commits](https://www.conventionalcommits.org/en/v1.0.0/) spec

---------

Co-authored-by: Anton Kaliaev <anton.kalyaev@gmail.com>
(cherry picked from commit 95af24d)