Short strings in lua proto are reused most of the time. The string in SSM may be added by one service and used by other service, there is no need to call `add_string`,which will call 'new_string' first.
This is a modify version of lua 5.3.2 (http://www.lua.org/ftp/lua-5.3.2.tar.gz) .
For detail ,
Shared Proto : http://lua-users.org/lists/lua-l/2014-03/msg00489.html
Shared short string table : http://blog.codingnow.com/2015/08/lua_vm_share_string.html
Signal for debug use : http://blog.codingnow.com/2015/03/skynet_signal.html