Remove the pnext pointer in CBlockIndex, and replace it with a
vBlockIndexByHeight vector (no effect on memory usage). pnext can
now be replaced by vBlockIndexByHeight[nHeight+1], but
FindBlockByHeight becomes constant-time.
This also means the entire mapBlockIndex structure and the block
index entries in it become purely blocktree-related data, and
independent from the currently active chain, potentially allowing
them to be protected by separate mutexes in the future.
* Bugfix: output the correct best block hash (during IBD, it can
differ from the actual current best block)
* Add height to output
* Add hash_serialized, which is a hash of the entire UTXO state.
Can be useful to compare two nodes.
* Add total_amount, the sum of all UTXOs' values.
