new flush_txpool command, and associated RPC call

It can flush a particular tx, or the whole pool (the RPC command
can flush a list of transactions too)

src/daemon/command_parser_executor.h

@@ -112,6 +112,8 @@ public:
   bool ban(const std::vector<std::string>& args);
 
   bool unban(const std::vector<std::string>& args);
+
+  bool flush_txpool(const std::vector<std::string>& args);
 };
 
 } // namespace daemonize