rpc: add blockchain disk size to getinfo

This should help new nodes predict how much disk space will be
needed for a full sync

src/blockchain_db/lmdb/db_lmdb.h

@@ -375,6 +375,8 @@ private:
 
   virtual bool is_read_only() const;
 
+  virtual uint64_t get_database_size() const;
+
   // fix up anything that may be wrong due to past bugs
   virtual void fixup();