Make root offsets chunk allocation configurable

[Chen-Yifan]

Add --root-offsets-chunk-count flag to monad_mpt to make the number of
chunks allocated for root offsets configurable (default: 16, must be
power of 2). Each chunk holds approximately 16.5M history entries,
allowing operators to adjust history depth based on deployment
requirements. Previously, this was hardcoded to 2 chunks.

[Copilot]

Pull request overview

This PR makes the number of chunks allocated for root offsets configurable through a new --root-offsets-chunk-count flag in the monad_mpt CLI tool. Previously hardcoded to 2 chunks, operators can now adjust history depth based on deployment requirements, with each chunk holding approximately 16.5M history entries.

Key changes:

• Added root_offsets_chunk_count configuration field with default value of 2
• Changed UpdateAux and UpdateAuxImpl constructors from taking pointer to reference for AsyncIO
• Updated storage pool to support configurable conventional chunk counts with backward compatibility

Reviewed changes

Copilot reviewed 18 out of 18 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
category/mpt/ondisk_db_config.hpp	Added root_offsets_chunk_count configuration field
category/async/storage_pool.hpp	Added num_cnv_chunks field to creation flags and metadata
category/async/storage_pool.cpp	Implemented configurable chunk allocation with backward compatibility
category/mpt/trie.hpp	Changed constructors to take AsyncIO by reference instead of pointer
category/mpt/update_aux.cpp	Updated set_io signature and chunk allocation logic
category/mpt/db.cpp	Updated constructor calls to pass AsyncIO by reference
category/mpt/find_notify_fiber.cpp	Changed member variable from pointer to reference
category/mpt/cli_tool_impl.cpp	Added --root-offsets-chunk-count CLI flag and validation
category/mpt/test/update_aux_test.cpp	Added test for configurable chunk allocation
category/mpt/test/*.cpp	Updated test code to use reference-based constructors

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