add --regtest and --fixed-difficulty for regression testing

on_generateblocks RPC call combines functionality from the on_getblocktemplate and on_submitblock RPC calls to allow rapid block creation. Difficulty is set permanently to 1 for regtest.
Makes use of FAKECHAIN network type, but takes hard fork heights from mainchain
Default reserve_size in generate_blocks RPC call is now 1. If it is 0, the following error occurs 'Failed to calculate offset for'.
Queries hard fork heights info of other network types

src/blockchain_db/lmdb/db_lmdb.cpp

@@ -1465,6 +1465,21 @@ std::vector<std::string> BlockchainLMDB::get_filenames() const
   return filenames;
 }
 
+bool BlockchainLMDB::remove_data_file(const std::string& folder) const
+{
+  const std::string filename = folder + "/data.mdb";
+  try
+  {
+    boost::filesystem::remove(filename);
+  }
+  catch (const std::exception &e)
+  {
+    MERROR("Failed to remove " << filename << ": " << e.what());
+    return false;
+  }
+  return true;
+}
+
 std::string BlockchainLMDB::get_db_name() const
 {
   LOG_PRINT_L3("BlockchainLMDB::" << __func__);

src/blockchain_db/lmdb/db_lmdb.h

@@ -181,6 +181,8 @@ public:
 
   virtual std::vector<std::string> get_filenames() const;
 
+  virtual bool remove_data_file(const std::string& folder) const;
+
   virtual std::string get_db_name() const;
 
   virtual bool lock();