added luasofia_su_root_step code and added destroy to meths table

I am trying to use the sofia root together with lua-handlers / lua-ev.
Currently i am using luasofia with lanes in a seperate thread but that isn't as nice as it can be.

I have added the su_run_step function, tested it and it seems to work as expected.

Also (in a lanes environment) it seems that the __gc call is not working reliable.
I am now using "destroy" with a set_finalizer from lanes.