Make HardFork object available to BlockchainDB and derived DB implementations

This will later allow the HardFork object's DB update functions to be
called when the DB transaction that persists across block add/remove is
open.

src/blockchain_db/berkeleydb/db_bdb.cpp

@@ -781,6 +781,8 @@ BlockchainBDB::BlockchainBDB(bool batch_transactions) :
     m_batch_transactions = batch_transactions;
     m_write_txn = nullptr;
     m_height = 0;
+
+    m_hardfork = nullptr;
 }
 
 void BlockchainBDB::open(const std::string& filename, const int db_flags)