add command pop_blocks

add new public method to Blockchain and update according to code review

update after review: better lock/unlock, try catch and coding style

src/daemon/rpc_command_executor.h

@@ -152,6 +152,8 @@ public:
   bool relay_tx(const std::string &txid);
 
   bool sync_info();
+
+  bool pop_blocks(uint64_t num_blocks);
 };
 
 } // namespace daemonize