stage_exec: allow more workers for build/collate/merge

[sudeepdino008]

i'm basicaly interested in making stage_exec/stage_custom_Trace/rebuild commitment faster by delaying/reducing the merge operation. Because now we do periodic merges there, and because we produce lot of files and end up "repeating" a lot of the merge.

in offline commands:
one thing was using optimal merge ranges - #19146
instead of periodic build+merge, we can do periodic BuildFilesInBackground(doMerge=false) and then 1 MergeLoop at the end after all executing is done-- I'll do it in some future PR. This ensures one-shot merge of (lots of) step1 files into a big file, rather than progressive merging into bigger and bigger files - which is repeated work.
integration stage_exec currently uses 1 worker for build/merge; it can be more (e.g. spawn_custom_trace use more workers) -- #19207 is meant to increase the number of workers for stage_exec (other offline commands look okay).

I don't intend to change behavior when erigon binary is running - chaintip or no chaintip. Just changing behaviour for offline commands with these changes.

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• not entirely happy with the solution (lock/unlock workers editing). Needed because ExecV3 modifies the workers.
• future: spend less time in merging in offline commands (just perform one at the end) -- will make stage_exec/spawn_custom_trace spend less time in redundant merges

[AskAlexSharov]

sorry, don't understand this PR