blockchain_db: pass hard fork object as a simple pointer

src/blockchain_db/blockchain_db.cpp

@@ -139,7 +139,7 @@ uint64_t BlockchainDB::add_block( const block& blk
   return prev_height;
 }
 
-void BlockchainDB::set_hard_fork(HardFork*& hf)
+void BlockchainDB::set_hard_fork(HardFork* hf)
 {
   m_hardfork = hf;
 }

src/blockchain_db/blockchain_db.h

@@ -377,7 +377,7 @@ public:
   virtual void block_txn_stop() = 0;
   virtual void block_txn_abort() = 0;
 
-  virtual void set_hard_fork(HardFork*& hf);
+  virtual void set_hard_fork(HardFork* hf);
 
   // adds a block with the given metadata to the top of the blockchain, returns the new height
   // NOTE: subclass implementations of this (or the functions it calls) need