Improve gettxoutsetinfo command

* 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.
Merge pull request #2602 from sipa/utxostate

Improve gettxoutsetinfo command