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

tests/unit_tests/hardfork.cpp

@@ -50,6 +50,7 @@ public:
   virtual void safesyncmode(const bool onoff) {}
   virtual void reset() {}
   virtual std::vector<std::string> get_filenames() const { return std::vector<std::string>(); }
+  virtual bool remove_data_file(const std::string& folder) const { return true; }
   virtual std::string get_db_name() const { return std::string(); }
   virtual bool lock() { return true; }
   virtual void unlock() { }