feature: new function get_commands that can return all commands

in the high level we need the all commands

maybe like this: 
https://github.com/h4lflife/lua-resty-redis/commit/a64c4a249804b2b87a0f16c2f3af6ac525c43481#diff-80ff59da1f1e7fce52f2bba643b14c76R396

we do not need to use the metatable (__index = function ) if we got the all commands
it can be more efficient