headerKeyCache: use otter v2

[maxm86545]

Benchmarks show that version 1, while extremely fast with hot keys, becomes several times slower than no‑cache under frequent misses. Version 2 delivers consistently better performance across all scenarios, with no allocations and stable latency.

BenchmarkGetUnCommonHeaderNoCacheSequential-12                           7545640               169.4 ns/op            72 B/op          4 allocs/op
BenchmarkGetUnCommonHeaderV2Sequential-12                               14471982                85.98 ns/op            0 B/op          0 allocs/op
BenchmarkGetUnCommonHeaderV1Sequential-12                               19748048                59.63 ns/op            0 B/op          0 allocs/op

BenchmarkGetUnCommonHeaderNoCacheParallelOneKey-12                      24352088                44.47 ns/op           72 B/op          4 allocs/op
BenchmarkGetUnCommonHeaderV2ParallelOneKey-12                           91024160                11.76 ns/op            0 B/op          0 allocs/op
BenchmarkGetUnCommonHeaderV1ParallelOneKey-12                           192048842                6.186 ns/op           0 B/op          0 allocs/op

BenchmarkGetUnCommonHeaderNoCacheParallelRandomMaximumSize-12           26261611                43.07 ns/op           62 B/op          3 allocs/op
BenchmarkGetUnCommonHeaderV2ParallelRandomMaximumSize-12                100000000               14.43 ns/op            0 B/op          0 allocs/op
BenchmarkGetUnCommonHeaderV1ParallelRandomMaximumSize-12                137813384                8.965 ns/op           0 B/op          0 allocs/op

BenchmarkGetUnCommonHeaderNoCacheParallelRandomLenKeys-12               24224664                46.57 ns/op           71 B/op          3 allocs/op
BenchmarkGetUnCommonHeaderV2ParallelRandomLenKeys-12                    69002575                17.42 ns/op            0 B/op          0 allocs/op
BenchmarkGetUnCommonHeaderV1ParallelRandomLenKeys-12                     8498404               253.1 ns/op            42 B/op          1 allocs/op

[dunglas]

Thank you!