Move HardFork DB update to BlockchainDB::add_block()

Ensures the database is consistent.

Also simplifes blockchain_import in that verify mode off has less to
work around.

src/blockchain_db/blockchain_db.cpp

@@ -130,6 +130,7 @@ uint64_t BlockchainDB::add_block( const block& blk
   // DB's new height based on this added block is only incremented after this
   // function returns, so height() here returns the new previous height.
   uint64_t prev_height = height();
+  m_hardfork->add(blk, prev_height);
 
   block_txn_stop();