The in-memory block tree, mapping each block hash to its index entry.
Declared in <node/blockstorage.h>
[[guarded_by(0x7ff37af16308), guarded_by(0x7ff376b832e8), guarded_by(0x7ff37ded0ed8), guarded_by(0x7ff36c6160f8), guarded_by(0x7ff37c499f58), guarded_by(0x7ff39283c958)]]
BlockMap m_block_index;