Handle to the on-disk block index database (blocks/index/).
Declared in <node/blockstorage.h>
[[guarded_by(0x7fbc8f17aa98), guarded_by(0x7fbca3f03fc8), guarded_by(0x7fbcace16218), guarded_by(0x7fbca6c57e88), guarded_by(0x7fbcad35f628), guarded_by(0x7fbc9fba13e8)]]
std::unique_ptr<kernel::BlockTreeDB> m_block_tree_db;