BlockchainBDB: Support blockchain_import --drop-hard-fork command

See c657e772c4 for LMDB equivalent.

src/blockchain_db/berkeleydb/db_bdb.cpp

@@ -1073,8 +1073,10 @@ void BlockchainBDB::sync()
 
         m_spent_keys->sync(0);
 
-        m_hf_starting_heights->sync(0);
-        m_hf_versions->sync(0);
+        if (m_hf_starting_heights != nullptr)
+          m_hf_starting_heights->sync(0);
+        if (m_hf_versions != nullptr)
+          m_hf_versions->sync(0);
 
         m_properties->sync(0);
     }
@@ -2213,7 +2215,37 @@ void BlockchainBDB::check_hard_fork_info()
 
 void BlockchainBDB::drop_hard_fork_info()
 {
-  /* TODO */
+  LOG_PRINT_L3("BlockchainBDB::" << __func__);
+  check_open();
+
+  bdb_txn_safe txn;
+  bdb_txn_safe* txn_ptr = &txn;
+  if (m_write_txn)
+    txn_ptr = m_write_txn;
+  else
+  {
+    if (m_env->txn_begin(NULL, txn, 0))
+      throw0(DB_ERROR("Failed to create a transaction for the db"));
+  }
+
+  try
+  {
+    m_hf_starting_heights->close(0);
+    m_hf_versions->close(0);
+    m_hf_starting_heights = nullptr;
+    m_hf_versions = nullptr;
+    if (m_env->dbremove(*txn_ptr, BDB_HF_STARTING_HEIGHTS, NULL, 0) != 0)
+      LOG_ERROR("Error removing hf_starting_heights");
+    if (m_env->dbremove(*txn_ptr, BDB_HF_VERSIONS, NULL, 0) != 0)
+      LOG_ERROR("Error removing hf_versions");
+
+    if (!m_write_txn)
+      txn.commit();
+  }
+  catch (const std::exception& e)
+  {
+    throw0(DB_ERROR(std::string("Failed to drop hard fork info: ").append(e.what()).c_str()));
+  }
 }
 
 void BlockchainBDB::set_hard_fork_version(uint64_t height, uint8_t version)