plugin: Introduce plugin type to allow singleton and chaining

The newly introduced type is used to determine what the call semantics of the
hook are. We have `single` corresponding to the old behavior, as well as
`chain` which allows multiple plugins to register for the hook, and they are
then called sequentially (if all plugins return `{"result": "continue"}`) or
exit the chain if the hook event was handled.

lightningd/jsonrpc.c

@@ -753,7 +753,8 @@ rpc_command_hook_callback(struct rpc_command_hook_payload *p,
 	                         "Bad response to 'rpc_command' hook."));
 }
 
-REGISTER_PLUGIN_HOOK(rpc_command, rpc_command_hook_callback,
+REGISTER_PLUGIN_HOOK(rpc_command, PLUGIN_HOOK_SINGLE,
+		     rpc_command_hook_callback,
                      struct rpc_command_hook_payload *,
                      rpc_command_hook_serialize,
                      struct rpc_command_hook_payload *);