add snax.self and snax.exit

test/pingserver.lua

@@ -1,5 +1,6 @@
 local skynet = require "skynet"
 local queue = require "skynet.queue"
+local snax = require "snax"
 
 local i = 0
 local hello = "hello"
@@ -32,6 +33,10 @@ function accept.hello()
 	end)
 end
 
+function accept.exit(...)
+	snax.exit(...)
+end
+
 function response.error()
 	error "throw an error"
 end