Merge: impacting chain-tip

[AskAlexSharov]

Problems:

• commitment.kv merge producing much storage.kv refferences read. Need try move to MMAP with MADV_WILLNEED
• Bloomfilters are stored in RAM (instead mmap). And during merge we can store 2x of Bllomfilters. Such "short peak memory jump" impacting ChainTip
• Recsplit.build() -> buildWithWorkers -> GolombRice.Append stored in RAM. Need move to Bufio or MMAP. (2g on some bloatnet file). Such "short peak memory jump" impacting ChainTip. Bloatnet got OOM because of this [performance] OOM on storage .ef indexing #20560
• Recsplit.build() -> buildOffsetsEf -> NewEliasFano -> EliasFano.deriveFields storing in RAM. Need move to Bufio or MMAP. (4g on some bloatnet file). Such "short peak memory jump" impacting ChainTip. Bloatnet got OOM because of this [performance] OOM on storage .ef indexing #20560
• .ef files merge have several in-mem buffers/arrays. For example one of them: db/state: inv_index merge remove 1 intermediate buffer to reduce peak memory #20597
• domain.collate: double-buffering remove: domain.collate: remove double buffering #20742
• post mortem on 32G validator: Merge post-mortem #18483
• Commitment.Domain deref: released files on main (for 3.6)
• BTindex.Build(): greedy EliadFano btindex: use off-heap EliasFano during build #21777
• BTindex.Build(): greedy keys copy

FYI: recent Merge-related PR's #19430

[VBulikov]

yperbasis added Imp1 as Importance